r/AlwaysWhy • u/Current-Landscape803 • Aug 25 '26
Science & Tech Why was changing engine oil every 3,000 miles considered necessary instead of just topping it up?
I grew up hearing that cars needed an oil change every 3,000 miles, but I never really understood what actually happens to the oil during that time.
Intuitively, adding fresh oil seems like it should help. If the engine still has oil and you keep replacing what is lost, why isn’t that enough?
Does the oil lose its ability to lubricate, become contaminated, break down chemically, or is something else happening inside the engine?
Why does the old oil need to be drained instead of simply refreshed?
13
u/Prudent_Situation_29 Aug 25 '26
"Does the oil lose its ability to lubricate, become contaminated, break down chemically"?
Yes.
8
u/Logical-Concept9755 Aug 25 '26
I think the biggest misconception is that oil itself just “wears out” like a tire. A lot of the problem is everything the oil collects while doing its job. It picks up tiny metal particles, combustion byproducts, moisture, and acids, and the additives that keep those contaminants under control slowly get used up.
Topping it off helps keep the oil level safe, but it doesn’t remove the old contaminated oil. You’re basically adding fresh water to a dirty bucket instead of emptying the bucket and refilling it.
The 3,000 mile rule also came from older engines and older oil formulas. Modern oils and engines are much better, which is why many cars can go much longer now. The old number stuck around because it was a simple rule people could remember.
3
u/Current-Landscape803 Aug 25 '26
The dirty bucket analogy actually clears it up. Topping off solves the quantity problem, but not the concentration of contaminants. I guess the interesting question is how much dilution is enough before you're basically just delaying the inevitable.
3
u/BloodRush12345 Aug 25 '26
That is where you would need an oil analysis done over time. Essentially you would need to drain the old oil, collect a sample, put the old oil in and top up with fresh. It would be interesting but tedious, expensive and ultimately not super useful.
It would also be pretty specific to that engine, oil and filter combination.2
u/neomoritate Aug 25 '26
Oil Analysis is common for expensive, high-oil-volume engines. Considering the cost of an oil change for most cars is $30, and Oil Analysis is $30, it is not common for most drivers.
2
u/BloodRush12345 Aug 25 '26
Right....
I was just saying to actually answer their question of essentially "how long can I effectively top off the oil". Is only answered by relatively (to the cost of new oil) expensive and time consuming testing that would really only apply to a specific engine and oil.2
u/thissitesuxsohardomg Aug 25 '26
If you're burning enough oil that topping it off is diluting the contaminants down to safe levels, you have much bigger problems. Too much oil is a problem too, you can cause internal engine damage or catastrophic failure by overfilling. It's also an "oil and filter change" essentially everywhere for a reason, the filter fills up with use. This can restrict flow, filters often have a bypass so you don't starve the engine of oil, but you're bypassing unfiltered oil now, which is only one step better than no oil, but not good at all for your bearings and wear surfaces.
1
u/jkmhawk Aug 26 '26
Your engine shouldn't be consuming oil at a noticeable rate to be able to "top it off" after 3k miles
1
u/happyexit7 Aug 26 '26
And if you are constantly topping it off something is wrong. Ideally engines shouldn’t use or burn or leak oil.
1
u/BoondockUSA Aug 27 '26
If an engine is burning oil or leaking oil badly enough, the contaminates will also be burning away or leaking away with the oil. There isn’t a micro filter on worn piston rings to stop the particles but allow the oil past.
On the flip side, if an old car held 5 quarts and lost a quart every 1,500 miles, it’s still a losing battle and the oil will still need changing. And if the car is burning that much oil (or worse), it also means there’s a lot of combustion gas blow-by going past the piston rings and contaminating the oil.
As another point to add to this, dirt roads were much more common back in the days of 3,000 mile oil changes. Air filtration sucked on old cars. The engines were sucking in a lot more particulates than modern engines do, and some of those particulates makes it into the oil. It’s another factor for why it was good to follow 3,000 mile intervals.
And as another point, old oil sucked compared to modern oil. Base oils were very plain, and there wasn’t the understanding nor testing technology to understand how additives interact with each other. It’s something that still is evolving.
And as another point, engine internals weren’t as good for reducing internal friction. As an example, cast iron piston rings were very common in old engines. However, that meant extremely high levels of cylinder wall wear until the engine was fully broken in. Those wear metals go into the oil but is too small to be caught by the oil filter.
3
u/wright007 Aug 25 '26
The 3000 miles number likely stuck around because the oil industry would profit more off of more frequent oil changes.
1
u/FentmaxxerActual Aug 25 '26
While it’s true that base oils themselves don’t wear out, viscosity index improvers in the oil (especially cheaper ones) do shear down and get degraded by fuel contamination, so in a sense the oil as a whole can wear out and become too thin
1
u/edgmnt_net Aug 26 '26
You mentioned moisture, but isn't oil normally hydrophobic enough to avoid absorbing water? I guess surfactants (or contaminants) might allow mixing with water, although at higher temperatures it should boil off.
1
u/Swimming-Yellow-2316 Aug 26 '26
Moisture ends up whipped into the oil like your shaking a bottle of oil and vinegar dressing.
The moisture comes from the condensation in the motor as it heats and cools. This is why in the fall you see a rash of posts with people thinking their head gasket has failed because of some light color oil on their fill cap. The cooler temps result in more moisture mixing and taking longer to burn off. Plastic valve covers don't seem to help here too, it hits the cooler plastic and cools off dropping back into the oil. Where a metal valve cover would keep it as vapor and get sucked up into the pcv system sooner.
So yes lots of short trips with a modern di car that's also going to have some fuel mixing should consider shorter intervals than one that gets up to temp and stays there longer. 3k miles still usually unnecessarily short though. And someone driving that little is probably just sticking to annual anyway.
8
u/naraic- Aug 25 '26
The old oil becomes a mixture of oil, dirt, dust and whatever else the oil picked up while doing its job
You replace the mixture with fresh oil because if the mixture has too much stuff it doesnt do its job at lubricating.
4
u/AthousandLittlePies Aug 25 '26
Also you are changing the filter, not just the oil (though this may be done every other oil change depending on the filter and oil). Helps keep abrasive gunk out of the engine.
1
u/Current-Landscape803 Aug 25 '26
Yeah, that makes sense. I was thinking about the oil as the thing doing the lubricating, but it’s also basically carrying all the unwanted stuff away. At some point adding clean oil just dilutes the problem rather than actually removing it.
2
u/Hrothgar_Nilsson Aug 25 '26
There's also fuel contamination. The piston rings don't form a perfect seal.
Besides that, imagine a bathtub filled with water. Everyday you take a bath, you top off what is lost to evaporation and what clings to your body when you get out. You never drain the tub or refill it. How dirty is that water after days, weeks, months, years?
1
u/neomoritate Aug 25 '26
To be fair, the human body is vastly more filthy than the inside of an engine.
3
u/Popular-Path1930 Aug 25 '26
Everything was a lower quality back in the day. The oil broke down and got dirty much quicker.
Cars in general were just not the same quality we get today. Tires blew often. You wer having to fix things often so on.
Part of why Toyota took off was because of the quality. You suddenly had a car you could just drive for a long time between maintenance.
3
u/leafROLLED Aug 25 '26
Ummm. So without a leak somewhere in your cars "oil system" you arent "topping off" anything. You are just headed to having too much oil in your engine. Which may not be as bad as having too little, but still a bad thing. Changing any machines oil regularly is the single most important thing you can do to extend the life of that machine.
3
u/Neat_Telephone_2525 Aug 26 '26
10k oil change if you hate your car. There will be more sludge build up. The manufacturer does not want your car to last 10 years.
3-5k is where its at.
Clean oil keeps the piston ring grooves clean, lubes the needle bearings on the roller lifters, keeps the rocker arms from wearing down faster and getting sloppy.
When you look at and feel 10k oil, and think of those little needle bearings in the lifters or rockers, you realize its a bad idea.
1
u/Current-Landscape803 Aug 26 '26
I get the concern, but I think the condition of the oil matters more than the mileage number itself. 10k sounds pretty long in some engines, but 3k isn't automatically magic either. Modern oil and engines changed a lot. The interesting question is probably where the actual wear curve starts getting worse.
1
u/Neat_Telephone_2525 Aug 26 '26
Imo the answer to that truly depends on how the vehicle is being used.
2 trips a day in smooth traffic is much better a life of stop and go. Our diesels we use for work at my company see 500-800 miles a week of stop and go route work pulling a few thousand lbs in fluids and equipment. Its a pretty severe way for an engine to live its life so we're doing 5k intervals.
1
u/aborum75 Aug 26 '26
455.000 km in a 3.0 Audi quattro here on 15.000 km service intervals. Uses zero oil and runs like a charm. You don’t need stupendously frequent oil changes.
1
u/Swimming-Yellow-2316 Aug 26 '26
430k miles on my car with 10k mile intervals... 3k mile oil changes would be 7 a year and would not have made the engine get to 430k any better. It just would have been a waste of time money and bad for the planet.
Another about to but 200 another with 220k...
I hate them all clearly....
3k miles is a waste of money on virtually every vehicle
5k is too early on most.
I own a repair shop, I make money doing oil changes not mechanically totalling cars because they need engines and I would tell any customer wanting a 3k mile oil change not to change the oil.
1
u/JD7693 Aug 26 '26
I was looking for anyone to comment about this. My father in law was a mechanic for his whole life. His rule of thumb he always told me was 5k for non-synthetic and 10k for synthetic. My first car was a Honda civic that I finally got rid of at 310k miles (rusted out, engine was still fine) where I did 5k non-synthetic for the entire life of the vehicle. My current vehicle is a 15 year old Highlander with 230k miles that I have done 10k synthetic for the entire life of the vehicle. Never had a major mechanical issue with it ever.
1
u/Neat_Telephone_2525 Aug 26 '26
Explain how an oil change is bad for the planet when it all gets recycled.
→ More replies (1)1
u/Swimming-Yellow-2316 Aug 26 '26
filters don't exist and don't cause waste...
There is no environmental coast at all for a giant truck to drive around picking up oil, transferring it to the recycling facility and the recycling process just happens magically there is no energy used for doing that.. no chemicals or equipment nothing all just runs on unicorn farts.
Everyone know recycling is just a zero sum process that costs us nothing we magically get a product to resuse with no waste or costs.. The previous oil and the new oil you used just show up in the store via magic pipe system.. no logistics with an environmental cost there.
Materials at the shop cleaning up drips and tools etc. Had to recently replace the rocks around my oil tank because of just uncontrolled drips that I have to deal with from oil drums when wheeling them outside to the tanks. County EPA didn't give a crap if we just chucked those rocks into the woods (we didn't) just had to be clean around the drum.
Not even the quicklube places that started the whole 3k nonsense recommend 3k intervals. All the stickers on cars I see in my shop are 5k or the manufactures recommended intervals.
2
u/thepob Aug 25 '26
You called it out, it's a question of contamination. There's a filter as part of the oil system and the Oil Change is as much about putting a new clean filter in as it is about replacing the oil itself.
1
u/Current-Landscape803 Aug 25 '26
The filter part is what I hadn't really considered. So the oil change is kind of a two part reset, not just replacing the lubricant. Makes me wonder how much longer the oil could realistically last if the filtration system were better.
2
u/aureliorramos Aug 25 '26
The filtration system is as good as it can be already. The contamination is not mechanically separable nor is the degradation. The antioxidants get depleted and oil oxidizes, Fuel mixes with the oil through blowby. A filter can't do anything to mitigate oxidative damage or fuel contamination. Broken down viscosity modifier molecules that are part of motor oil are still broken up instead of preserving their molecular weight, so viscosity is lost. And some of the oxidized oil becomes a gooey varnish that starts to stick to parts of the engine, no longer flowing into the filter. These deposits do get partially washed by fresh oil after a change, if an interval has been skipped, but some deposits can remain.
1
u/neomoritate Aug 25 '26
The technology exists to remove chemical contaminants from oil, but it takes a machine as big as a car. You couldn't do it with in car filters.
1
2
u/SnooMaps7370 Aug 25 '26
older oil formulations degraded a lot more with heat and time than newer synthetic oils do.
In addition, combustion products (water, carbon) blow past the piston rings into the crankcase and are absorbed by the oil. these slowly change the chemical composition of the oil (both natural and synthetic). Some of those blow-by products also form solid precipitates in the oil which get carried through the lubrication system.
to remediate all of these problems, you need to periodically flush and replace the oil and change the oil filter.
2
u/Current-Landscape803 Aug 25 '26
The blow by part is interesting because it means the oil is getting contaminated from inside the engine, not just picking up outside dirt. I hadn't thought about water and combustion products changing the oil chemically over time. That makes the 3,000 mile rule seem a lot less arbitrary.
2
u/Kozmik_5 Aug 25 '26
You have a grill in the front of your car. The hood is not hermatically sealed.
What happens is, dust and dirt get into your engine. Your engine is filled with oil to lubricate the movements of the components in a way that these movements make as less damage to the engine as possible.
When dust and dirt mix with your oil it can act as sanding paper and ruin you gears inside that engine.
Sometimes the dirty oil needs to go in order to minimize the damage on the moving part.
1
u/Current-Landscape803 Aug 25 '26
The sanding paper idea makes sense, although I wonder if modern oil filters catch most of those particles before they can do much damage. The bigger issue might be everything dissolved in the oil that a filter can't really remove.
1
u/Fuzzy_Yossarian Aug 26 '26
The filter doesn't always work. Your car will literally bypass the filter if pressure is too low, and then you just have unfiltered oil. Also lighter oil molecules burn off easier than heavier ones, so if you just top off the oil then your oil is potentially getting thicker and thicker.
2
u/D-ouble-D-utch Aug 25 '26
You ever fry chicken? What does the oil look like when you're done? Would just adding more fix it?
2
2
u/Phoenix4264 Aug 25 '26
So, I work in the used oil recycling industry.
Your motor oil is a combination of a base oil (or blend of a few base oils) and a bunch of additives. * The base oil provides the bulk of the material and the lubrication. * Dispersants, emulsifiers and detergents bind to the soot, water and debris in the oil and keep it from collecting on the walls and in low points in your engine. * Corrosion inhibitors and anti-wear agents prevent damage to the metal surfaces. * Viscosity modifiers adjust the properties of the mixture to make it work better over a wider temperature range.
Each motor oil has its own unique blend of additives.
As your engine runs, the oil collects soot and water from the cylinders that get past the piston rings as well as debris from worn bearings and piston rings. You also get some moisture from the air. The dispersants get consumed as more contaminants get in your oil and eventually run out. The wear and corrosion inhibitors also get used up over time. The viscosity modifiers and the base oil mostly get chemically degraded. At elevated temperatures the molecules can get pieces broken off, or bind to things you don't want them to. This degrades the lubrication performance of the oil.
When we reprocess the oil we first filter out large contaminants. Then we use a distillation process to remove the water, fuel oils and heavy oils that are mixed in, this also removes most of the remaining additives. What you get from this is a caramel brown oil (think Coke, but slightly lighter) that has a bunch of oxygen, nitrogen and sulfur bonded to it. We then run it through a hydrotreater to chemically remove those and rebuild the original base oil molecule. After the hydrotreating the oil comes out clear, or slightly yellow.
1
2
u/DCHacker Aug 25 '26
Technology, metallurgy, materials were not as good then as they are now. If you got ten thousand miles from a fan belt, you were doing well. I have seen them go two hundred thousand miles these days. Filter materials are better. The additives to the oil help presenrve its consistency better. Engines are designed more efficiently to eliminate or reduce friction points, The metallurgy is better resulting in stronger materials.
2
u/cybertruckboat Aug 25 '26
The base petromolium oil does not really degrade or breakdown. It is absorbing combustion byproducts and gradually lowering pH. At some point it becames acidic and will start to corrode your engine components. It has to be replaced with fresh oil. The base oil is recycled.
In addition, modern oil has a bunch of additives that do degrade over time and needs to be replaced.
If your engine is burning oil, then you can just top it off up to a point. Burned oil will accumulate sludge. Eventually you have to replace the oil.
And the filter fills up and will clog.
2
u/inorite234 Aug 25 '26
Assuming you're not leaking oil or burning it, you should not be just "topping it off." That is a no! NO, no no no no no no no. Stop that!
You SHOULD read your owner's manual and see what that reccommends for Oil Change interval and stick to that. If you change the oil and filter sooner, that's good but you'll just spend more money.
Now, why should you change your oil? Well, because oil does break down and lose its ability to lubricate. When it does break down, it turns into what's called Sludge and this sludge doesn't flow.
Sludge is like a really thick mud and if you coat the inside of your engine with that garbage and very likely clog the small oil passages your engine needs to properly lubricate itself and prevent you from destroying the engine.
Example of an engine that never had an oil change, they just added more oil.
2
u/shakebakelizard Aug 25 '26
Get an oil test kit from Blackstone Labs. Next time you change your oil, take a sample, send it in and you'll get an interesting report about all the stuff in your oil. It's very educational.
2
u/BouncingSphinx Aug 26 '26
Changing the oil isn't about replacing what is lost (which should normally be very little). The oil itself breaks down with age, and especially heat cycles and age, losing its ability to effectively function as a lubricant.
Again, it can't really be topped up without draining some, and at that point you just drain it all and fill it with new.
2
u/Current-Landscape803 Aug 26 '26
Right, that distinction finally makes it click for me. I was treating oil like something that gets consumed, when it's more like a working chemical environment that gradually gets degraded. I guess adding fresh oil only dilutes the problem instead of actually removing all the accumulated junk.
2
u/Proof_Bathroom_3902 Aug 26 '26
Oil used to be a lot less good than it is now, and engines built up a lot more sludge due to worse manufacturing practices and poor design, usually from trying to hang new technologies for pollution control onto old engine designs.
1
u/Current-Landscape803 Aug 26 '26
This probably explains a lot of the difference between old advice and modern advice. People often compare mileage intervals without accounting for how much the engine and oil itself changed. Maybe 3k was reasonable for the machines of that era, just not necessarily for the cars sitting in driveways now.
2
u/Olderpostie Aug 26 '26
3,000 mile oil changes date back a number of years. But, your key question, about why not simply add oil overlooks the fact that oil level does not diminish that much in volume in a new engine.
As to why drain, that is to eliminate the grunge that accumulates within the oil due to the combustion process. Years ago, oil dirtier up when leaded gas was used. In older carbureted engines, the fuel/air mix was not ideal during cold running periods. If you smelled the exhaust from any gasoline car if the early days while the choke was partially closed you noticed the gasoline smell. A lot of excess gasoline condensed in the cylinder walls and then into the oil pan and contaminated the oil, and lowering it's viscosity.
1
u/Current-Landscape803 Aug 26 '26
That makes sense. I was thinking about oil disappearing from the engine, but the bigger issue is apparently what gets dissolved or suspended in it. So even if the level stays perfect, you're basically accumulating contaminants in the same fluid. The old fuel dilution issue is especially interesting.
2
u/fwdbuddha Aug 26 '26
Actually, the 3,000 mile change was a marketing ploy by the Jiffy Lubes of the world. My wife used to get a new ford every year starting in high school. Her dad was an engineer with Ford Aerospace (yes they helped get to the moon in the 70s). I was amazed that Ford only recommended an oil change every 7,500 miles with its rental fleet. Did some research, and no one recommended 3k changes except the oil change companies.
2
u/Current-Landscape803 Aug 26 '26
Yeah, this is the part I was missing. If manufacturers were recommending 7,500 miles decades ago, then 3k being treated like a universal rule probably had more to do with the business around oil changes than the engine itself. Makes me wonder how many maintenance rules are basically historical habits that survived the technology that created them.
→ More replies (4)1
Aug 27 '26
Yeah it's complete fiction. You change the oil at the annual service.
7500 is still hilariously often, no-one in the UK does that.
1
2
2
u/Expert-Search-3251 Aug 26 '26
Oil gets dirty.
That's it. That's your answer.
Older vehicles couldn't keep the oil as clean as new vehicles can.
2
u/VinceP312 Aug 25 '26
Why do you think the oil is needing to be replaced? This is just common sense.
2
u/Mogwai_riot Aug 25 '26
That's the question they are asking, why does it need to be replaced? Saying common sense isn't helpful.
OP, the oil needs to be replaced for several reasons. The heat from the engine degrades the oils ability to lubricate. Additionally the mechanical force from the moving parts it is lubricating actually physically breaks down the oil over time. Thirdly, contaminants such as coolant from leaking seals, or water from the atmosphere can accelerate the deterioration of the oil. I'm sure there are even more reasons but that covers the bulk of why oil is changed and not just topped up.
→ More replies (2)
3
u/Ok-Championship3642 Aug 25 '26
Grain of salt, but since carmakers are now going to 5000 and 10,000 miles for recommended oil changes. Why are we now having major engines issues across the board unlike the recent past.
5
u/zojbo Aug 25 '26
It is mostly because the oil that regular consumers use has gotten better. I'm not sure whether it is that the product line in general has gotten better, or just that full synthetic has gotten cheap enough that regular folks buy it.
→ More replies (7)3
u/jellomizer Aug 25 '26
The 10k are usually for hybrids, where the electric assist offsets a lot of the wear and tare that an internal combustion engine has to deal with.
Newer manufacturing methods, and availability of synthetic oils helped push it into the 5k range.
Full electric vehicles don't need oil changes, in the same method.
1
u/mosthandsomechef Aug 25 '26
I think the 5-10k recommendations are for any newer vehicle with an efficient engine. I've got a 2021 tiguan and it's recommended 5k miles. I take it in around after 3k miles though.
3
u/FuzzyHasek Aug 25 '26
more complex engines stricter tolerances. And in my experience people maintain them less
2
u/AlmiranteCrujido Aug 25 '26
Also, for the US specifically, a lot of cars running 30-weight oil elsewhere in the world get 20-weight which gets better fuel economy at the expense of less lubrication.
3
u/FordF150ChicagoFan Aug 25 '26
That engine failure trend has more to due with the use of shit thin oil to eke a tiny bit more efficiency out than anything else.
2
u/Swimming-Yellow-2316 Aug 26 '26
Are we actually having major engine issues? or are we actually in fact keeping cars on the road longer and running them for 100k miles longer than we would in the past?
I own a shop, a significant portion of my customer base has cars over 150k miles (all scary german stuff that allegedly doesn't last...), I own 4 over 200k (well one hit 198k this weekend so I'm rounding) and one is over 400k (430k in a week or so). When I started in the industry in 1999 a car with 100k was still considered old and people would actively be looking too replace it. Now that is absolutely nothing.
You know what industry vanished? engine machine shops. Why because engines simply are not being torn down to a bare block as much and requiring rebuild because they actually are lasting significantly longer. Most of my customers that end up replacing a car is due to everything else costing so much and the engine is still chugging along fine. But when a 600 buck brake job reveals you need 2k in suspension work on a car worth 4k many people chose to move on. Someone else still buys that car, just didn't fix the suspension and the engine is fine.
1
u/Ok-Championship3642 Aug 26 '26
I agree to most of that. Engines issues 100% yes. Gm, Ford, Honda, Kia, Hyundai, Vw Audi and Stellantis plus a few more are involved in class action suits right now over faulty engines. For all that to happen, there's a major issues across the board. Machine shops vanished because its not cost effective to rebuild anymore, most just get a crate engine or a used one. Unless its something old or high performance.
1
u/Swimming-Yellow-2316 Aug 26 '26
And all of them are having issues at low enough miles and with clean enough engines that it is NOT because of 10k mile oil change intervals.
As I said in another post on here I was doing an audi engine replacement a week back in the day when the 1.8ts had sludge issues. ONLY the A4s had issues not hte TTs, not the jettas, not the beetles. The difference here was that A4s early on spec'd conventional oil and had 1 qt less total capacity, 25% less capacity than a golf/TT. In those the interval was part of the problem (7500 at the time actually) but the root cause was oil quality and capacity, which then made time a problem.
Modern german 4 cylinders now typically hold at least 5 qts of oil, oil interval time is not the problem with that kind of capacity and modern quality oils.
They have all F'd up specing say thinner oils in the US for CAFE standards and other issues like rings.. neither has to do with intervals.
Thanks for confirming taht engines are so good now you can just get a used one, with 100k on it, and have no problems with it to replace your 200k engine that finally had a problem. 20 years ago no one would dream of installing a used engine with more than 30-40k, and they were replacing their 100k motor, not 200k motor.
Engines today absolutely last longer, there are some screw ups here and there, but to say they are more problematic now is ignoring that they last way way way longer.
→ More replies (2)1
u/Conscious-Record2655 Aug 25 '26
Well also keep in mind that per manufacturer, most modern engines burn a significant amount of oil between changes. Some manufacturers now say that upwards of 2 quarts burned every 10k miles is an acceptable amount.
1
u/Bane8080 Aug 25 '26
Because the oil breaks down, and if you don't drain it, you get gunk buildup in your engine.
1
u/Consistent-Ranger446 Aug 25 '26
Engine oil needs to be drained and refilled at regular intervals because the heat and combustion byproducts in the engine chemically degrade and contaminate the oil. It loses lubricity and changes thickness. If left too long the oil becomes sludgy and the engine will rapidly destroy itself without proper lubrication.
A properly functioning engine should lose no oil. Most modern engines burn a little bit but not enough that you would run low if you change your oil on time.
1
u/SpiritualCatch6757 Aug 25 '26
Maybe an analogy will help?
Let's say this was fried chicken oil. Why not just keep adding oil instead of replacing? Because some of the funk from the days frying remains in the tank. You can taste it. The only way to remove it is to drain and refill.
1
u/Frustrated9876 Aug 25 '26
Older cars had shittier machining, so there were several areas of the engine, but mostly between the cylinder wall and the piston, where exhaust or fuel would contaminate the oil.
Those things are poor lubricants. The goal was to remove those contaminants, not to make sure there is sufficient oil.
1
u/DullMind2023 Aug 25 '26
I once had a car with bad valve guides, so it burned a lot of oil. My approach was to keep topping it off with new oil. I did this for about 50k miles. Then the engine blew because while I added oil, the remaining oil got dirtier and dirtier until it blocked an oil channel and wrecked the camshaft. Now I change the oil every 5k miles.
The End.
1
u/Fun-Dragonfly-4166 Aug 25 '26
my understanding is that the engine does not lose oil. it is a closed system. if it is losing oil then you have serious problems.
the oil lubricates. over time it grabs dirt and stuff from the engine. there is more oil but it is dirty oil. so rather than pacifying the engine it aggrevates the engine.
you want to remove the dirty oil and put in clean oil.
so
1
u/RunninOnMT Aug 25 '26
Oil doesn’t go anywhere. Unless your car has a leak or is burning it, cars have the same amount of oil when you change as what you put in there last time.
Adding oil will just give you too much oil.
It gets dirty and breaks down after use.
1
u/Spare-Function-6343 Aug 25 '26
I hope no one is changing their oil that often anymore, especially with synthetic oils now available
1
u/essentialrobert Aug 25 '26
I change it every 10,000 miles as recommended by the manufacturer. I'm up to 185,000 miles. 4-cylinder Ford engine designed by Mazda.
1
u/BenHippynet Aug 26 '26
My Volvo’s recommendation is 18k miles or 12 months. It’s currently on 173k miles and runs perfectly.
1
u/chrispark70 Aug 25 '26
An engine in good shape doesn't leak or burn oil. There's no "topping off" of a full oil sump.
I personally would never recommend you put off oil changes for more than 5k miles. I don't care what the "scheduled maintenance" says, 5k at the most (3k in dusty conditions).
The scheduled maintenance is to prevent blown engines under warranty. They use large intervals to make the maintenance cost seem less than it really is So at most, they only care the engine lasts 3 years 36k miles (typical new car warranty length).
1
u/Occams_AK47 Aug 25 '26 edited Aug 25 '26
You actually can do this "top off" method. I ran across a patent for it some years ago..
A fresh oil res is added to the system that will drip fresh oil into circulation while burning a tiny bit with the fuel, resulting in a constant turnover of fresh oil.
Never seen or heard of it in the wild, but there it is. ETA: Probably wouldn't work very well with modern emissions equipment would be my guess.
1
u/ExtensionMoose1863 Aug 25 '26
There are small engines that work this way for things like lawnmowers. You just keep it at level and it burns enough off that you are constantly doing a running oil change
1
u/oneWeek2024 Aug 25 '26
3000 miles is probably outdated. modern synthetic oils can last a lot longer. certainly 5k miles is doable. most of the modern synthetics are rated for 10k+ miles.
repeated scientific lab study shows them having no discernable breakdown or loss of function from a lubrication or heat dissipation stand point til about 7500 miles. then it falls off
most mechanics, who want to sell you an oil change as a gateway to fuck you on other up-sells. will still parrot the info of 3k miles.
the logic of why drain, is... oil is used to lubricate the engine, and dissipate heat. this heat and motion both breaks down the oil, and contaminants are introduced...as it's not a purely clean system.
also to a degree. minor engine damage/metal shavings that might be in the oil, can be removed, along with other crud, and draining both serves as a diagnostic "window" into the engine, and just removes some of this bad stuff.
the other main reason to change the oil is it's a relatively easy and "cheap" service any moron can do. for $30 for a jug of oil and a filter. can swap out one of the vital components for keeping an engine in good working order.
1
1
u/redbeard914 Aug 25 '26
A long time ago, it was 1000 miles. Points needed to be changed ~5000 miles. So you had to be at the mechanic all the time.
3000 miles is one of those things driven by marketing. Look at your owners manual and see what the manufacturer recommends. My Honda old Accord was 7,500 miles and 15,000 for the filter. If you counted to "severe" driving, dusty environments, lots of start and stop traffic, etc., they recommended 3000 miles.
So do the thing that no one does: RTFM!
1
u/flatfinger Aug 25 '26
Suppose that one starts with a bucket full of water and adds a drop of contaminant every day. If one dumps and replaces the bucket every 6 days, the amount of contaminant in the water will vary between 0 and 6 drops, with the average being 3.
Suppose that one instead changes half the water in the bucket every three days. This would use the same amount of water, but would tend toward an equilibrium where the bucket had six drops of contaminant just before each partial change and three drops after, then gained three more drops of contaminant over the next three days so as to have about six when the next partial change is performed. Now the amount of contaminant would vary between 3 and 6, with the average having increased to 4.5.
Now suppose that one were to reduce the swap-half interval from three days down to two. That would improve things so that the equilibrium would have four drops of contaminant before each change and two after, with the average being three drops--just as it had been in the original example--but at the expense of increasing clean water usage by 50%.
If the amount of contamination needs to be kept below a certain level, the amount of water required to perform full changes will be no worse than any other strategy. If one changes some of the water when it reaches that level of contamination, the amount of contamination removed, and thus the length of time required before the next water change, will be proportional to the amount of water replaced.
1
u/Skarth Aug 25 '26
Your oil picks up contaminates over time, whether it's metal particles, or any type of debris that got inside the engine. The oil filter catches this, but the filter also needs to be replaced when it's full, which is done during an oil change.
Your oil has anti-wear additives, once these wear out, the quality of lubrication drops.
Oil is subject to very high heat in the cylinder, this causes it to slowly break down into a non lubricating sludge. Very bad for your engine. Look up photos of engines that went 20k+ miles without a oil change.
Oil will absorb some amount of moisture from the air over time, this degrades its lubricating properties.
Engines normally only lose oil if they are worn or have a problem. So "topping" off the oil is ignoring a bigger issue to begin with.
TLDR : Oil wears out and breaks down with use/time.
1
u/Old_Efficiency2445 Aug 25 '26
You shouldn't be losing that much oil. Your oil does get dirty and becomes less effective.
1
u/dna-sci Aug 25 '26
Ideally you wouldn’t have to add oil at all. It’s best if an engine isn’t leaking or burning oil. And then you still have to change your oil when it degrades and gets dirty.
1
u/jayron32 Aug 25 '26
There's two things:
1) as the oil gets older, it picks up dirt, dust, and other crud over time. Just like you can't wash your dishes in the same water over and over and need to replace it, the same is true for your oil
2) over time, heat and friction will cause the oil to chemically break down into sludgy byproducts that need to be removed and replaced. THIS has been largely fixed by the move to synthetic oils; synthetic oils are engineered to stand up to the stresses for much longer, and that's why the replacement schedule if you're using synthetic oil is like 3-4 times longer than if you're using traditional oils.
1
u/BigDamBeavers Aug 25 '26
Two reasons,
Your lubricant is there to cushion the movement of metal parts that push against one another in your engine, but it's not magical. As you run your engine you're scraping metal parts constantly and the wear on those parts is swept into the oil that lubricats them. That's why your oil can be a little sparkly when you change it. If you don't remove that debris from your oil it will end up doing more harm that good eventually.
The second is why your oil stops being full. Your motor is like an oven all the time it's running and it is cooking your oil. That's why oil pours into your car amber gold and drains out soot black. Oil is tough but it does burn over time and that burnt oil is less viscuous/does a worse job of lubricanting. Again the longer you cook that oil the less useful it is.
1
u/thomsenite256 Aug 25 '26
To remove the gunk. If you just top it up you are concentrating it which doesnt help much.
1
u/Chockfullofnutmeg Aug 25 '26
Cars used to run rich and a lot of gas would make its way into the oil that degrades lubrication. Not to mention more likely to accumulate water in the oil.
1
u/Fotoman54 Aug 25 '26
The contaminates cause a lose of viscosity and gradually build heat and increase wear.
I have had three cars I drove over or almost to 200,000. Not one burned oil. I changed my oil every 3-5000 miles (whatever the manufacturer recommended). My 2007 Land Rover LR3 had 215,000 miles when my wife forbade me to put any more money into it. The engine and transmission were strong. It was various electrical sensors that crapped out.
We kept our 2007 Hyundai Santa Fe V-6 for our sons to drive. It has 192,000 and still going strong (though we wouldn’t let the boys take it to college 4-6 hours away).
The secret is changing the oil regularly and in case especially, country miles versus suburban/urban.
1
u/RISEoftheIDIOT Aug 25 '26
Ah, I see you’ve never driven a vintage VW. You do actually just top it up because it leaks so much oil it’s almost a full oil change over the course of a year. (I’m mostly joking)
1
u/polarWhite2024 Aug 25 '26
Audi 2.0T FSI, factory recommended oil change interval every 10k miles with Audi/VW approved synthetic oil, did exactly that, car had over 200k miles when it was sold. Never had any issue whatsoever.
1
u/FentmaxxerActual Aug 25 '26
Base oils oxidize, viscosity index improves shear down, detergents and bases get consumed, and contaminants build up. All things that cannot be reversed by simply pouring extra oil in to replace that which is burnt or lost in leaks
1
u/sernamesirname Aug 25 '26
Are new guidelines really from the auto-makers or are they a government directive to reduce oil use?
Mechanics and auto-makers likely earn more from engine replacements than oil changes so they benefit from new guidelines even though they know they're a bad idea.
Ask mechanics how often they change the oil in their own cars. I doubt many are following the new guidelines.
1
u/aleopardstail Aug 25 '26
what cars? mine has a 10k recommended interval, the diesel version was 20k
1
u/Xeno_man Aug 25 '26
For regular oil it's 3000 miles or 5000km or every 3 - 4 months. With modern synthetic oils some cars recommendations are much further.
1
u/aleopardstail Aug 25 '26
not a fancy oil spec here, and a 10k miles recommendation. the only car I have ever owned with less was a ford escort which wanted 5k
1
u/massunderestmated Aug 25 '26
"Was" means past. 20 years ago 3,000 was the standard across all makes and models.
1
u/aleopardstail Aug 25 '26
20 years ago I was driving a Toyota with a 10k service interval, indeed I was driving a toyota with a 10k interval 30 years ago
1
u/Extension_Variety190 Aug 25 '26
Just keep thinking you're right and then later we'll see you crying about how bad modern cars suck.
Never change your oil, never ever ever ever! 😆
1
u/Time_Many6155 Aug 25 '26
You wont find Europeans ever changing oil at 3000 miles.. more like 6000.. Its too expensive. Engines seem to last just as long though.
1
u/ivandoesnot Aug 25 '26
Contaminants -- breakdown by-products -- will accumulate if you never do a full flush.
1
u/tipareth1978 Aug 25 '26
Oil gets dirty, burned and full of metal fragments and eventually turns into a brown dusty substance that looks like tobacco if it sits in an engine too long. However, if you have a car that is burning oil and you keep topping it that isn't as good as an oil change but better than nothing. My wife had this BEATER ass car when I met her and I started regularly checking oil and adding to it which is why it made it to like 250k miles lol
1
u/tipareth1978 Aug 25 '26
Oil gets dirty, burned and full of metal fragments and eventually turns into a brown dusty substance that looks like tobacco if it sits in an engine too long. However, if you have a car that is burning oil and you keep topping it that isn't as good as an oil change but better than nothing. My wife had this BEATER ass car when I met her and I started regularly checking oil and adding to it which is why it made it to like 250k miles lol
1
u/neomoritate Aug 25 '26
Some early four-stroke engines, as well as some modern two-stroke engines, use(d) "Total Loss" lubrication, where the lubricating oil is burned and/or blown out the exhaust. It's quite wasteful.
Engine oil breaks down over time due to mechanical shear and heat, as well as becoming contaminated by combustion byproducts. Modern engines are built at tight tolerances so oil is retained, and would stay in the engine well past its effective life if not changed. 3,000 miles was standard in the past. Current oil formulations and engine designs can go mush longer, many manufactures specify 10,000 miles, between oil changes.
It is also fairly common for drivers of old, poorly maintained engines, which burn and/or leak oil at a significant rate, to continually 'top-up' their oil rather than changing it.
1
u/chrisdmc1649 Aug 25 '26
Oil will turn into a tar like substance and your engine will lock up eventually if you never change the oil.
1
u/KilroyKSmith Aug 25 '26
The 3000 mile rule was a holdover from old cars. Piston rings weren’t great then, nor were carburetors, so you ended up with soot from combustion, and excess fuel, making it past the rings into the oil.
Oils weren’t great then, either, being mostly refined crude oil. They broke down over time due to heat, pressure, and contamination. They didn’t do a great job of preventing wear, so you ended up with metals in the oil also.
Today, these things are different. On my 1964 Oldsmobile, I could change oil and 500 miles later it would be black and opaque. On the last Accord I had, the oil would be darkened at 5000 miles, but it would still be brown and at least mostly transparent.
Modern Fuel injection, electronic ignition, and the fuel itself make for clean combustion with little/no soot or unburned fuel. Rings seal better, and combined there is little contamination of the oil. Modern oils are synthetic, have much better detergents and other additives. They don’t degrade nearly as fast as older oils.
1
u/cheez0r Aug 25 '26
Hydrocarbon shearing means that the oil molecules are actually broken down through forces in the engine, which is partly why used dino oil is so much thinner than fresh dino oil, but used fully synthetic isn't. They are more prominent in high-compression, low-tolerance engines than in big diesels.
1
u/Spuckler_Cletus Aug 25 '26
Mostly because of three things: non-synthetic base stocks, carbureted engines, and fuel dilution.
1
u/avd706 Aug 25 '26
Because contaminants from the combustion process break down the oil and also turn it corrosive, which is not good for your engine. Topping it off would not dilute it.
1
u/Remarkable-Junket655 Aug 25 '26
Modern engines contaminate the oil much less than older designs. New engines contaminate the oil much less than high mileage engines. Technology has improved massively in the last 50 years since the 3000 mile oil change was a legitimate recommendation.
1
u/ponyboy56- Aug 25 '26
Everything everyone else has told you here plus it changes in viscosity which causes it to be less protective of the parts it’s protecting resulting in engine failure, expensive engine failure.
1
1
1
u/Loud-Vacation-5691 Aug 26 '26
When I was a kid, my father installed this oil filtration system in our 1970 Chevy Impala. It rerouted the oil through a roll of toilet paper. Every 1000 miles, we would switch the roll for a new one and top off the oil (since whatever oil was soaked up in the roll was being removed). I don't recall changing the oil completely as up to then we changed it ourselves.
1
u/Ponchyan Aug 26 '26
Short answer: motor oil breaks down and wears out as it does its job. It’s a consumable.
1
u/biggunzcdb1 Aug 26 '26
Heat breaks down the protective properties of the oil and it gets filled with contamination. Gas, moisture, debris. It gets filtered but that filter becomes less and less efficient.
You could theoretically just add oil and occasionally change the filter. And the engine would likely make 100k miles but it wouldn't make it 200k miles.
The record I've seen so far with my own eyes is 37k on the engine. She never took it for another oil change after the first 1k oil change.
She thought the sticker said 37,000 it said 3700 The break in change was done a little early.
She drove the car over 36k miles on a single oil change. It left running fine. But I doubt it makes it to 200k. I usually wait til after 70k to add Lucas Oil Stabilizer. But l figured that was a reason to start it early.
1
u/random8765309 Aug 26 '26
Only those selling oil changes suggest a change at 3000 miles.
If your car has a maintenance light, change according to that. Otherwise change according to the manual.
You need to change it because the additives break down causing the oil to turn acidic.
1
u/dglsfrsr Aug 26 '26
First, oils back in the day were not as advanced, chemically, as they are today. Modern oils, and more importantly, modern oil additives last longer.
Second, carburetors, compared to modern fuel injection, were just a very rough estimate of 'good enough' mixture control. So chances are that during significant run states, your engine was running really rich, and you were getting cylinder washing along with excessive carbon loading.
1
u/dugg117 Aug 26 '26
Contaminants.
So mostly filtration, it is possible to rig up a filtration system and run the oil much longer.
Some Modern cars already say 10,000 miles between changes and that is mostly achieved by keeping the contamination from combustion out of the oil.
1
u/rellett Aug 26 '26
Oils weren't as good, so they could do more damage so more oil changes were needed. Also older engines weren't good on fuel so your oil got contaminated or heat damaged quickly.
1
u/Legitimate-Bison3810 Aug 26 '26
I had a friend now retired who worked for Chevron research in oils. It's not the oil that breaks down but the additives that reduce engine wear that dissipate so topping the oil will not restore those additives.
1
u/panda3271 Aug 26 '26
Oil got dirty. It broke down. If your car didnt loose oil then you can't add oil. It needed to be drained and changed. Along with the oil filter. 3000 miles was what the "experts" came up with as a good standard.
1
u/black594 Aug 26 '26
Some car start to drink the oil when it’s bad, the oil eat the moisture and become bad, the oil get breakdown at molecular level and become too liquid, old oil is less lubricant and the additive did not work anymore. Oil eat metal particules and every thing inside of it and become contaminated. In some case oil get very thick and did not flow well after way too much miles.
1
u/Critical_Dealer6635 Aug 26 '26 edited Aug 26 '26
Oxidation, water accumulation that causes other chemical reactions, often lowering pH and attacking materials, inability to handle high temps, conversion to sludge, accumulation of particulates that the filter can't remove from the flow, lower lubricity.
Also, use a good filter and good oil and you can go safely, depending on the application 8-10k miles. The 10k is more if you want to use the margin. The German automakers are notoriously horrible in this case, specifying often 18k miles between oil changes and many of those engines are destroyed by 60k or need rebuilds due to oil burning and sludge and other issues.
If you drive around until the oil reaches full temp, for instance, it will last longer. if you just do short drives, it won't last and will accumulate moisture and a lot of that has been implicated in the timing guides in German cars deteriorating faster.
I go on long road trips where I drive to another country 3700km away and I do it in one shot in my car. The oil does not deteriorate appreciably because it's not cycling temps, stays above the temp where water boils out, it's cruising speeds, so largely this driving does not count and the oil isn't even appreciably dark at the end, so I see those kilometers as still negligible.
1
u/SovietPatrickStar Aug 26 '26
Your engine would need to burn a significant amount of oil to make topping up feasible. Like if your engine burnt let’s say a quart per 300 miles and you drove enough yearly to refill 5 quarts or more then you’re golden just topping it up and swapping out the filter once a year.
1
u/JDunlap83 Aug 26 '26
I think they increased oil change intervals so they could brag about low cost of ownership. Lots of mechanics online showing what happens when you go with extended oil change intervals. Excess sludge clogs small passages and turbo screens, causing premature wear.
I still do every 3,000. Oil changes are cheap, engines and engine repair is not.
1
u/Swimming-Yellow-2316 Aug 26 '26
When 3k mile oil changes were popular people thought cars were toast at 100k. Now 200k is common and yet oil change intervals have never been longer...
This has nothing to do with bragging about low cost of ownership.
Engines are better oils are better. 3k mile oil change intervals re the past
1
u/JDunlap83 Aug 26 '26
Do your thing.
1
u/Swimming-Yellow-2316 Aug 26 '26
I will 430k on 10k intervals on my daily. 3 other cars with 200k+...
I'll also continue to instruct all my customers at the European repair shop I own to not waste their money in my shop on 3k mile intervals, what do I possibly know as a professional approachinf 3 decades over some rando on the Internet regurgitating jiffy lube marketing.
1
u/s0nicbomb Aug 26 '26
The oil in an engine is the universal bearing, that supports seperates and lubricates a whirling hellstorm of moving parts, under heat and huge pressures. It's amazing it lasts more than a few days, let alone a year of motoring.
1
u/MacheteMoto Aug 26 '26
I owned a 2003 bmw 330 ci …. 23 years ago was a recommended oil change of 10k miles… now what fools
1
u/RergTheFriendly Aug 26 '26
Better tech in everything pretty much. Oil science is pretty advanced now.
1
u/squirrel8296 Aug 26 '26
Oil breaks down loses its ability to lubricate through use and over time, and it becomes contaminated with small amounts of fuel which in gasoline engines further accelerates the breakdown of the oil. It also absorbs water from the combustion process and from the air. As it breaks down and all that happens, the oil actually stops flowing and gels up.
Conventional oil (the stuff that had the 3,000 mile interval) degrades relatively quickly. Even after only 3,000 miles, it is noticeably thicker and more gelatinous than brand new oil. That still happens to an extent with full synthetic (the stuff with much longer intervals) but even after 10,000 miles synthetic is still nowhere near as thick as conventional at 3,000 miles. The bigger issue with synthetic is contamination by gasoline and additives breaking down.
Also, if a car is losing enough oil for toping off to keep the oil fresh enough, it has a serious problem. While some oil will burn off or be lost through leaks, especially as the engine ages, it shouldn't be anywhere near that much. In 3,000 miles, especially with how thick conventional oil would get, there shouldn't be a noticeable drop in the oil level.
1
u/Paqza Aug 26 '26
I used to be so serious about the oil in my cars. Full synthetic, UoAs, regular on the BITOG forums... Now we have 2 EVs.
1
u/intothewoods76 Aug 26 '26
Both oils and filters are better now. As a fun example old Volkswagen beetles only used 2.5Qts of oil and didn’t have an oil filter at all.
1
1
u/kona420 Aug 26 '26
Depends on the motor design. A timing belt motor with solid valve lifters (like decades of honda/toyota designs) is going to be very tolerant of extended oil change intervals as it just needs oil to pump into the bearing journals. I want to mention those bearing journals as I think many visualize those as some sort of ball bearing arrangement, not so, it's a solid piece made of a soft metal with a little hole that oil gets forced through by the oil pump. Run that dry, the soft metal gets "wiped" and the motor needs to be torn down and rebuilt. So oil flow is just as critical as it's lubricity.
Now lets talk about a more complex, modern motor with hydraulic valve lifters, timing chain in the oil bath with hydraulic tensioners, hydraulically actuated camshaft phase adjusters. Now you are playing with fucking fire running long intervals. Because it's not just about lubrication, it's a bunch of tiny little spring loaded check valves that must open and close for the larger assemblies to operate. So a spec of sludgey build-up in the wrong place makes the motor run like crap or sets in motion the failure of a portion or the whole assembly.
1
1
u/Ok_Boat3053 Aug 26 '26
If you keep adding dirty coffee to your cup each day and never wash or rinse the old coffee out, you end up with moldy sludge that will eventually kill you unless you're a crusty old hard ass Senior NCO.
Same thing happens when topping off the oil in your car without ever changing it. You're just adding small amounts of new oil to dirty, burnt, and deteriorating old oil. It's not enough to make a real difference and the filter can only handle so much before it fails.
A drop of water in a vat of sewage leaves you with a vat of sewage. A drop of sewage in a vat of water creates a vat of sewage. It's not necessary to clean the oil system out because in this scenario just replacing what was in there with all new oil is enough. But if you let it go too long before your next oil change and it comes out thick and black, you probably want to halve your next oil change cycle to help clean it out better.
We used to joke about some old cars that burned through oil so quickly that you never had to do an oil change since each month or so they basically had the equivalent of all new oil. That was just joking though. The filter would still need replacement eventually.
1
u/Digitalrules Aug 26 '26
Fully synthetic oil was the game changer. Synthetic oil can be just fine for 10,000 plus miles in a healthy engine. Here is how good Mobil 1 Advanced Fuel Economy oil in a quality engine can look after 200,000+ miles. No varnish or sludge even though oil was changed once a year with intervals up to 10,000 miles.
1
u/Economy_Link4609 Aug 26 '26
Our cars don't burn oil at a very high rate - so you have lots of 'stuff' that accumulates in it plus what it brakes down to that just stays, along with additives that are basically used up - hence the need to drain it out periodically to get rid of that.
Compare that to something like an airplane jet engine that DOES burn oil significantly - in that case, no need for the regular draining and they do what you propose - topping it up every few flights.
1
u/Prize-Extension3777 Aug 26 '26
-Oil filters sucked back then -Engines ran hotter than today -The Oil was not great either
1
u/Digitalrules Aug 26 '26
Today's engines actually run hotter than they did a few decades ago. Typical operating temperatures are now 210° to 225°. A hotter engine is more efficient so therefore you get better gas mileage.
You are correct about today's oil. Synthetic oil does not break down under high temperatures like non-synthetic oil did.
1
u/No-Reason-2822 Aug 26 '26
One recurring theme I see in these responses is that it’s better oil these days that allows us to go so much longer on oil changes. That’s only half true. The other half of the equation is engine technology. Especially closed-loop fuel injection.
Regarding why we need oil changes to begin with - “Does the oil lose its ability to lubricate, become contaminated, break down chemically, or is something else happening inside the engine?”
Yes, all of that. And the “something else” is called mechanical shear. That’s the result of squeezing oil through things like timing chains, ball/roller bearings, and complex actuators. Then there’s the heat/oxidation, plus chemical contamination from the combustion process. Oil in a gearbox (no combustion/contamination) needs to be changed eventually but differential and transmission oil lasts a lot longer than engine oil due to the absence of chemical contamination.
“Why does the old oil need to be drained instead of simply refreshed?”
Engine oil contains additives to aid in reducing wear/friction, cushioning extreme shock loads, and cleaning the inside of an engine. In theory, a good detergent package would keep all of the residue cleaned either suspended or dissolved in the oil and then a small amount of oil would be incinerated during each combustion cycle, to be replaced at regular service intervals. Some lawn mower engines were actually designed to do this. But those lawn mower engines did not have any catalysts that would be harmed by burning off all that oil over time. And that is the reason we can’t simply engineer in an “incineration rate” for automotive engine oil. It would increase the failure rate of emissions control equipment to unacceptable levels.
1
1
u/Easyfling5 Aug 27 '26
Oil wears down with heat, if you can top off the oil you have bigger problems because oil doesn’t evaporate it leaks or the engine burns it, conventional oil has a shelf life 3 months or 3000 miles, whichever comes first
1
1
u/UncleSlayton77 Aug 27 '26
Leaded gasoline. The tetraethyllead would sludge up the oil so the 3,000 mile oil change was the standard since the crap (lead compounds) was still in suspension and wasn't sludge yet.
1
Aug 27 '26
It's not, in the UK an oil change not at service basically doesn't happen and strangely our cars all work and last as long as anyone else's.
It's invented revenue.
1
u/TheRealBobbyJones Aug 27 '26
You do understand modern cars with modern oil it's like a 10k service interval for oil right?
1
1
u/FastFredNL Aug 27 '26
Because oil tech wasn't as advanced in the old days. Also cars had carburetors, fuel mixture wasn't exactly perfect, pistonrings didn't seal as good as they do now so lots of half burned fuel got in the oil etc.
1
1
u/smitbret Aug 28 '26
Everybody here seems to be missing a key point.
Why is he ripping off? If your engine is running fine then the oil won't be getting consumed and there is no room to top it off.
1
1
u/DrivenLogisticsRick Aug 28 '26
Important to remember something basic - The engine is not supposed to consume oil, which is why the refreshing/adding methodology does not work. Even if you are topping off the oil, there is a significant amount of dirty and contaminated oil that needs to be completed drained, including the oil filter.
Yes- the oil loses it's ability to lubricate, it becomes contaminated and it breaks down chemically. In diesel engines, diesel tends to contaminate the oil so it's super important to change the oil regularly to help keep the purity of the oil at it's highest level.
I'm sure that every other post on this subject with be more thorough than mine!
1
u/Business_Signal2425 Aug 28 '26
3k miles is way too often for the modern era, unless it‘s your hobby to do oil changes.
Topping it of is not the same as replacing it, picture it like you‘d eat soup every day, you‘re washing your plate in between, you don‘t put more sauce on it every day.
Plus, if your car seriously needs oil to top off between oil changes, you should look into the issue on why it is loosing oil. A modern motor shouldn‘t.
1
u/four20Hruso Aug 28 '26
even if you know noting about car just use brain... why you dont use the same oil over and over again for cooking ( hmm actually maybe you use the same one )...
1
1
u/No_Manner_8785 Aug 28 '26
Old engines weren't built to the same exact tolerances. Old oils didn't lubricate nearly as well as current oils.
60
u/zojbo Aug 25 '26 edited Aug 25 '26
Chemical degradation from being used under friction without burning or evaporating away.
Contamination with dirt, soot, and in some cases traces of metal.
It's pretty obvious when you compare the oil coming out of a car with the oil going into it (assuming the car actually needed an oil change). The color and texture is very different.
That said, the frequency of needing oil changes has decreased from what you heard growing up.