HOXXOH

The OLM measures the conditions that cause the additives in the oil to lose their effectiveness. Miles driven are meaningless, since at 60 MPH, which takes one minute, you've traveled one mile and the engine rotated 1300 times. By the same token, if you sit in your driveway for two minutes at a 650 RPM idle, you've added 0 miles and the engine rotated 1300 times.

0% on the OLM is what GM has determined through testing, is the half-life of the additives. It isn't even close to causing engine damage. The actual oil will still continue to lubricate everything, even when all the additives have been depleted, but just not with the same features the additives provide. The additive half-life is the safety point for oil changes.

It's like having 8/32 of tread on new tires and you know they are half gone at 4/32. You can still drive, but the tires don't work as well. When you get to 2/32 the wear bars clearly show and you know the end is near. You still can drive, but it starts getting scary and finally the cord shows and you're headed for a blowout. Well, the 4/32 was the half-life of the tires and many people change them around that time, just to be safer.

Bruze

This thread is a great example of how the computer age gives us TONS of data -- a huge multiple of what was available before.

What I learned as a young-un a half-century ago still stands today:

- Change oil and filter at recommended intervals (changing the oil and NOT the filter always seemed pointless to me, and I've never done it. I don't think they recommend it anymore . . . ?) When I was young I changed at 2,000 miles but that's not necessary for most of us today.

- But if most of your driving is short trips, where the oil may not reach operating temperature, you should change more often -- although I guess the modern % meter takes that into account.

- Remember, the oil level is not like the fuel level -- run it down then fill it up (I had a GF in an earlier life who thought that). Keep it near the full level. If you notice it dropping regularly then you have a problem.

- Always change oil and filter before storage.

Not So Fast

Let me rephrase my question, anyone have a engineers or oil analysis report as to what happens to oil after one year with miles driven under the allotted amount?
Lots of theories but just wondering what scientific reports might say.
Thanks
NSF PS Yes I will Google it

LMB-Z

I change mine when it gets below 10%. I don't drive the car hard at all, so I see no reason to change it any sooner. Never had an engine failure in 45 years of driving. Change it when you want, its your car.

Bruze

According what I learned way back, and I don't think this has changed, is that oil accumulates acids and other by-products of combustion (which the tranny and diff oil are not exposed to).

So it's not a great idea to store or run an engine like that.

How much difference does it make? Probably not much I suspect. Instead of needing a rebuild at 250k miles maybe you'd need one at 200k or something. It won't cause sudden, massive failure of the engine.

LDB

There are three main villains with regard to oil life: sludge, acidity, and oxidation. Sludge is mainly what turns the oil black, and is mainly condensed hydrocarbons from piston blowby. Assuming your rings are in decent shape, you haven’t disconnected your PCV system, and you don’t leave your oil in for ridiculous intervals, this should not be much of a problem. As an aside, I might mention that disconnecting your PCV system is the worst single thing you can do from both an air pollution standpoint and also from an engine wear standpoint. The reason it’s so bad for engine wear is that it means your crankcase is pure piston blowby vapor which is corrosive and degrades the oil, where with PCV operational, the piston blowby vapor is diluted to low levels by the air drawn in by PCV.

Back to the story, the other two issues (acidity and oxidation) are related, because the carboxylic acids that can eventually form contain oxygen atoms, so initial oxidation is a necessary step along the route to acid formation. Oils have both anti-oxidants and pH stabilizers to prevent this from happening, but eventually those become depleted. The main feature of extended life oil is having more of those additives so that you can drive more miles before they are depleted. But reverse logic does not apply. Having an excess of those additives if you change your oil often does not give you any added protection. So it would not make sense, for example, to use extended life oil and then change it every 3000 miles, figuring that would give you added protection. Incidentally, while oxidation and carboxylic acids may cause the oil to discolor a bit toward brown, they will not turn it black, so a non-black dipstick does not necessarily equate to ok oil.

The problem in answering the original question (how often to change oil) is that so many factors influence how rapidly the degradation from sludge, acidity, and oxidation occur. How well are your rings and seals performing? How hard do you drive? Are there any odd environmental factors like unusually dusty, hot, or pulling heavy loads? And the biggest of all, how often do you shut the car down before it is thoroughly hot, recognizing that damage from that practice gets exponentially worse if you have several semi-warm shutdowns in a row before getting it thoroughly hot. Semi-warm shutdown is important because that leads to condensed water vapor getting in the oil, which seriously aggravates oxidation and acid formation. Is time itself a problem, meaning change it after X months or years regardless of mileage? If you don‘t have any of the above oil stress factors, time itself isn’t an issue until at least 1 or probably more like 2 years. Similarly, if you don’t have any of those stress factors, there’s no reason not to run the oil life monitor all the way down.

I know I haven’t given you an exact answer, but that’s because no exact answer exists. The answer is everybody’s favorite, namely, “it depends”. Hopefully, the above gives you at least a sense of what it depends on.

LDB

Used oil analyses are pretty much worthless. Yes, if you are about to have a catastrophic engine failure, they will show elevated metals. Yes, if you’ve left your oil in way beyond anti-oxidant and pH stabilizer depletion they will show low pH and high oxidation products. But your engine is toast way before such signs show up. For anything resembling a normal engine, there is far more random noise variation from sample to sample than there is true, underlying effect. If used oil analyses were reliable and useful, no oil companies would do the very expensive engine tests needed to assess wear properly. But they do such testing, which conclusively shows how useless used engine oil tests are.

HOXXOH

Good question NSF. Let's preface with a little history.

After WWII car manufacturers got more concerned about oil filtration, oil lubrication, and warranties. To compensate for the ability of the general public to travel more often and farther, they created better criteria to determine service schedules. Oil changes among other things were tied to mileage as the primary measure. Because not all owners kept a log of miles driven the only other method was time. The early stuff was 3,000 miles or 3 months, whichever came first. Later on, when positive lubrication and full flow filtration became standard, they kept the 3,000 miles, but switched the 3 months to one year. As both the engines and oils got better, the mileage increased, but the one year remained as a reminder for those who could remember annual events like birthdays, holidays, and car purchases, but never paid attention to mileage. Now that we have a much better system to identify oil capabilities than just an odometer, we still don't have a way other than a calendar to jog the memories of people who ignore maintenance, which is why the one year deal stays in the OM. Annual simply provides a way for manufacturers to assure that at least some maintenance has been done during the warranty time period.

Because current engines are essentially sealed to the atmosphere when not running, the oil is not subjected to conditions that cause oil degradation. The result is that it's not necessary to change oil annually, except for warranty purposes.

Follow the OLM and you'll never have a problem.

jrose7004

One year or 10,000 miles for me. But I never get to 10,000 in a year.

LMB-C6

Yes, and no. As far as the condition of the engine, I agree that by the time the analyses shows elevated traces of metal, or trace amounts of antifreeze, etc. that the damage is done (or is being done), but I think it does have some value in telling you what's going on with your oil.

I had a DD that I used Mobil 1 in for years, I'd change the oil around 5K-6K miles. I have a very short commute, and I didn't always follow the rule that I needed to change the oil at least every year. So that's 2 bad things going on: 1) short commute that didn't get the engine nice and hot and 2) I was keeping the oil in there beyond the 1yr 'limit'. I decided to get an oil analyses to see if anything bad would come up...and the results surprised me. They said the oil looked good and they suggested I could safely prolong the service interval yet another 2K miles. I know this is a sample size of 1, but in my case running a full synthetic another 6 months (sometimes even more than that) with short commutes in the cold North East had no adverse effects. I sold the car after 15 years of ownership, it's still running around town.

That said I change it every year in the Vette, not because it will cause damage if I don't, but because spending that $35 gives me piece of mind.

Bruze

No, there are no obvious "adverse effects." As I previously stated, it will not cause a sudden engine failure.

When I was young and buying junkers all the time, whenever I took the valve covers or oil pan off, sometimes there would be a whitish/grayish sludge all over everything. It could be 1/4 to 1/2" thick as I recall. Usually you could just take the oil fill cap off and see the sludge below it in the valve train area. It was one of the first things I checked when looking at a car I might buy.

Changing the oil at that point, which I did on every car I bought, was a good idea but it had no effect on the sludge -- it stayed where it was.

Someone older and wiser than me (don't remember who) told me that the sludge was from the previous owner making short runs, especially in the winter, when the oil didn't fully warm up. Changing it more frequently under those conditions would have helped. I'm sticking with this idea until proven false.

Other cars could have 100k plus miles and the interior of the engine looked like new because the owners didn't make short trips.

So no, there probably aren't any obvious adverse effects, and it may not even show up in an oil analysis, but who wants that sludge covering the interior of the engine? Although I've never really heard of this happening, what if a piece of sludge came loose and plugged an oil passage?

65 Drivr

I have not yet exceeded the oil change interval from the cars computer which I reset after each oil change.
With that being said, I change my Mobil 1 oil once a year in my 08 LS3.
I have been also using Mobil 1 in all of my other many vehicles for the last 25 years (give or take a few years) and have been doing the same once a year oil change. I don't think I have ever gone more than 7 or 8K miles in a year on any vehicle. I do like to bring their engines close to the redline on occasion and have not lost a motor yet so I guess I'm ok at once a year.

CMC6

Mobil 1 every 5K for me