Thanks, yep see you have a impressive background. What I stated re monitoring oil temp after cold starts comes from a fellow who used to post here. He worked with the GM engineers who developed the OLM. He stopped posting when too many back yard mechanics told him he did not know what he was talking about.

Could be something else but wonder if it could be starting, running for a short time and pulling it back in the garage. Or just starting for 5 minutes periodically, far worse than just leaving it sit. For the 8.2 Liter engine I built and put in my Street Rod I was changing oil as the Chevy Crate engine info said MINIMUM every 12 months. But now I go to few local Shows/year. It stays in the garage. IF I have to get a ladder it is blocking, I make it a point NOT to just pull it out, get the ladder and put right back. I have a ~25 miles route I drive to get the oil hot and get the water of combustion (lots with those large Forged NOT Cast pistons like the C8) out of the oil. I now do NOT change every year.

Built in 2000 when I retired (well semi, still have a part time Internet business.) I followed the 8.2 Liter crate motor instructions and changed oil every 12 months. The last 10 years it only goes to a few local shows/year. So I avoid ever just pulling it out to get a ladder it is blocking. Always drive >25 miles.

Yep Street Rod now parked against the far wall blocking ladders etc. I do not pull it out unless I drive ~25 miles. Has been sitting ~6 months not started! It will be fine, AGM Yellow Top Optima Pugged into my GM type Charger.


Yep, just keep my 25 year old Street Rod plugged in when not at Shows. Back out to get something, drive ~25 miles to get oil hot long enough to evaporate water of combustion that pasted the cold pistons.

 

You can see in his post #18 responding to me. He changed his oil 4 to 5 months ago! The car has then been stored. I asked if perhaps it had been started periodically for 5 to 10 minutes that would have not gotten the oil up to temp to evaporate water. The OLM may have reduced the 12 months. Will see if he responds.

 

My bet is that he's mistaken about how long ago he had the oil changed. Don't see how even starting periodically could have cut that much time off of the OLM.

 

Perhaps, but GM did not put this in the 1993 Corvette Owners Manual (before there was an OLM in Vettes) because it was a major issue!

Since some may not want to bother the fine print is says Change Oil Every 3 Months IF:

• Most trips are less than 4 miles
  • If below freezing if trips are less than 10 miles
• Most trips involve extended idling and/or frequent low speed operation in stop-and-go-traffic
• If vehicle is used for delivery service, Police or Taxi service

A Page From My 1993 Vette Owner's Manual BEFORE an OLM Existed.
Had to use what was never a good oil change indicator but all we had, Time and Miles. Time, even today's 12 months, requires changing because of contamination (yep even for Garage Queens with 1500 miles!) . Cold Starts are the worse thing for an engine and oil. Water of combustion passes the cold pistons. If the oil does not get Hot enough, Long enough to evaporate much of that water it combines with things like Sulfur in oil and gasoline (that also pass the cold pistons) and forms, in that case, Sulfuric Acid. That Eats metal when the car is parked.
The OLM monitors Oil Temp after starting and if it does not get hot enough long enough will shorten the 12 months minimum change to less. The OLM algorithm knows far better than I just how many times that occurs and estimate how long to shorten the time. Before the OLM it was a guess and with DI, 3 months would appear to me to be excessive. But GM engineers know better than me!

 

Similar information resides in the C8 manuals as well, though I do not know it would explain how the remaining oil life dropped to 14% in 5 months (if that is accurate). But anyway, an updated version, is still found in the 2025 Owner's Manual but now it says you can rely on the oil life indicator. Also, as I read the bulletin that explains, at a very high level, the factors in the OIl Life System, its says that cold or hot temperatures, will shorten the engine revolution pathway depending on how far engine temperature is from the normal operating temperature. So, that will only have an impact if there are engine revolutions. Its not clear that affects the time pathway. We don't know the exact details about how the impact of time and the other factors is combined in the algorithm. But what we do know is the manual says to change the oil when the OLM says so.
Here is the link to the 2025 version of the bulletin:
https://static.nhtsa.gov/odi/tsbs/20...01327-0001.pdf
Heres from the 2025 Owners Manual Severe Service Section:
"Severe Conditions Requiring More
Frequent Maintenance*
• Public service, military, or commercial use
vehicles to include the following:
– Ambulances, police cars, and
emergency rescue vehicles.
– Civilian vehicles such as light
duty pick-up trucks, SUVs, and
passenger cars that are used in
military applications.
– Recovery vehicles such as tow trucks
and flatbed single vehicle carriers or
any vehicle that is consistently used
in towing trailers or other loads.
– High use commercial vehicles such
as courier delivery vehicles, private
security patrol vehicles, or any
vehicles that operate on a 24–
hour basis.
– Any vehicle consistently operated
in a high sand or dust environment
such as those used on oil pipelines
and similar applications.
• Vehicles that are regularly used for short
trips of 6 km (4 mi) or less.
The oil life indicator will show you when
to change the oil and filter. Under severe
conditions the indicator may come on
before 12 000 km (7,500 mi).

 

^^^
Yep as I have said my 1st Vette, a C6, with an OLM and the second two C7s, simply said use the OLM. No mention of miles and time was also accounted for in the two C7s. The C6 required, like my two C4's before the OLM you to keep track of the calendar. BUT it said must change a minimum of every 12 months!

I still blame GM for confounding the 7500 miles for a DCT Filter change with Oil Life miles. Makes folks think of the 1970s and oil company adds thinking miles is a good indicator of when to change. Never was! The OLM considers miles, total engine revolutions, measures oil temp (especially important after cold starts) etc elc. Also blame them for eliminating the Owner's Manual Maintenance Table after 2020 and 2021. For some, they only thing they read! It clearly said only the OLM matters but now have to hunt to be sure: DID I MISS SOMETHING!

PS: Still poorly written as it says under sever conations it may come on before 7500 miles BUT forgets time! Foolish. Makes folks think miles is key when the short trips have to do with contamination NOT miles.

Nope reminds me of the foolish way my 1st C7 standard shift moved the seat when I entered. It moved right before your eyes as soon as the door opened. Had to be faster than a speeding bullet to use. I'm convinced that seat software was written overseas by someone not old enough to drive!

Yep when I opened the door in my 1st C7 the seat moved to my memorized location. Instantly, right before your eyes, like the invisible man was sitting in the seat. Had to be faster than a speeding bullet to use.


I was convinced the software that had worked great on my C6 had poorly written instruction for the person overseas they outsourced to who never drove a car. Was fixed the next year eliminating that foolish move as soon as door opens!

 

Yeah, there is a bit of a mystery there. While the manual never says and never said to change the oil at 7,500 miles it did say to check it at 7,500 miles and change if necessary. But we know from the bulletin (and other sources) that the Oil Life System is capped at 7,500 miles and 1 year - it will never go beyond those without indicated an oil change. As for the DCT filter change requirement at 7,500 miles - its interesting to look at Owner's Manual for the Maserati MC20, which uses exactly the same transmission. It requires a filter change at 9320, and every 9320 miles after that. To me, that suggests a bit of leeway in exactly when the first filter change needs to be (not from a warranty point of view, but technically). So, I can speculate (and others before me have as well) that GM picked 7,500 miles for the nominal filter change mileage so it would correspond to the cap on the oil change interval. Unfortunately that does not work out for most of us that drive less than 7,500 miles per year. It is interesting the GM insists on change the transmission fluid between 7000-8000 miles even if done earlier, so much so that they send a letter to all owners about it, and yet the MC20 can go to 9320. Does the MC20 have a bigger filter????

When GM eliminated the table, they could have put all the requirements in simple sentences located in one place - but did not. Something like this (let me know if I messed up):

Oil Chnage: Change the engine oil and filter when you get the Change Oil Soon message on the DIC within the next 600 miles but before the indicator reaches 0%. If the Oil Life System is accidentally reset, change the oil at 3000 miles from the last oil change. The engine oil must be changed once a year, regardless of mileage. Reset the oil life system when the oil is changed.

DCT Fitler Change: Change the DCT external canister filter when you get the Replace Transmission Filter Soon message(not all vehicles have this) before the indicator reaches 0%, at 7,500 miles +/- 500 miles even if changed before, at 22,500 miles, then every 22,500 miles after, or every 24 hours of track use, whichever comes first. Reset the filter life system (if you have it) when the filter is changed.

Transmission Fluid Change: Replace the Transmission Fluid when you get the Change Transmission Fluid Soon message within the next 600 miles but before the indicator reaches 0%, or 45000 miles, or at least every three years, or every 24 hours of track use, whichever comes first. If the fluid life system is accidentally reset, change the fluid 45000 miles or three years from the last change, whichever is first. The fluid must be changed at least every three years regardless of mileage. Reset the fluid life system when the fluid is changed.

 

^^^
Yep agree whoever is writing these Owner's Manual does not consider the audience. I'm convinced it's certainly not Corvette management. Recall in a Steve Garrett Podcast Interview John Holder was apologetic confounding the first oil change with the 7500 mile DCT filter requirement. Camera panned to Harlan Charles who smiled. Yep they were defending whoever. Similar to defending the GM Bean Counters who removed the mechanical key back-up from the Truck! Their Job!

Water in Oil
Know I emphasis the water of combustion that passes the cold pistons at start as causing moisture in the oil requiring an oil change if not quickly evaporated. Have searched my files for the fellow who worked with the GM Engineers who developed the OLM who said that was a key issue. But I only have his long diatribe that centers around ZDP depletion (dialkyldithiophosphate which is an anti-wear compound comprised of zinc and phosphorus.) It's so long won't post!

This what he stated in one paragraph:

"The other things that determine oil life such a acid build up, oxidation, petane insuluables such as silicon from dust/dirt, carbon or soot build up from the EGR in blowby, water contamination, fuel contamination, etc.... are all modeled by the multipliers or deterioration factors that "adjust" the immediate slope of the line defined by the engine revolution counter as those items can be modeled in other ways and accounted for in the immediate slope of the ZDP depletion line. "

So in addition to what I learned in the mid 1950s from my Uncle, (Service Manager at a large Chevy dealer, later GM) who said it’s NOT miles but the type that are important. Are they Good Highway or Poor City miles? I asked Google.

Know that is far from perfect but it can search many thousand Docs in seconds. Here is what it retuned

My question of water of combustion getting past cold pistons at starts:

AI: After a cold start, water in engine oil will typically evaporate as the engine warms up to its normal operating temperature:

Also Searched water of combustion forms with sulfur in oil after cold starts

AI: Yes, during cold starts, water of combustion can form with sulfur present in fuel and oil, leading to the formation of sulfuric acid which can cause corrosion.

Here's a more detailed explanation:

· Combustion and Sulfur:
When fuel containing sulfur is burned in an engine, some of the sulfur is converted into sulfur oxides (SOx).

· Sulfuric Acid Formation:
These sulfur oxides, particularly sulfur trioxide (SO3), readily react with water vapor (both in the atmosphere and in combustion gases) to form sulfuric acid (H2SO4)<<

· Cold Starts and Condensation:
During cold starts, the engine's temperature is low, and moisture can condense on the inside of the cylinders and crankcase.

· Corrosion Risk:
This condensed water can then combine with the sulfur oxides to form sulfuric acid, which is a corrosive substance that can damage engine components.

· Oil Breakdown:
The sulfuric acid can also break down the lubricating qualities of the oil, leading to increased wear and tear on engine parts.

 

For those interested in the detail in the algorithm, I suggest turning to the actual technical paper, written by the engineers(s) that developed the algorithm while working at GM. That avoids any errors that make their way into other "diatribes" that may or may not get it right. Granted, the algorithm has for sure evolved since its creation in the 80s to adjust for change in engine technology and lubrication technology. There is also a list of subsequent publications for the interested reader. And there is the 2025 bulletin that give a high level description of the algorithm.

1987 technical paper by Shirley E. Schwartz and Donald J. Smolenski
https://sci-hub.se/10.4271/870403

Additional papers by Schwartz et. al.:
http://profiles.sae.org/shirley_e._schwartz/

The recent bulletin
https://static.nhtsa.gov/odi/tsbs/20...02441-0001.pdf

 

My bet is that he forgot to reset the OLM when he changed the oil before storage.

@JerryU , here's the explanation of the OLM you're looking for that has been posted here multiple times.

 

Yep, glad you posted so I don't get criticized for having long posts!

But that fellow had many long posts, that one focused on ZAP antiwear additive measurements. He had a number of others posts one that discussed measuring oil temp after cold starts and IF it did not get Hot enough Long enough to evaporate much of the water of combustion it would shorten the 12-month minimum time to change. He also said a number of things can lower the 12 months could reduce the time BUT NOTHING will increase.

BTW he stopped posting because too many would-be mechanics did not believe his info and posted critical comments!

My 1993 Vette Owner's Manual validated the short trip issue were the oil dodes NOT get Hot enough.


But you're right could be longer than he remembers. We'll see if he answers.

 

Thats the issue I have with that version of the explanation - while its not necessarily incorrect - its focus is on the ZDDP additive and its depletion. Its why I continue to suggests folks read the actual paper by the inventors - it is a more thorough and complete explanation of the algorithm which does include but is not limited to ZDDP depletion. I did not post it because there is some language about permissions and fees - but it is free to view at the link.

https://sci-hub.se/10.4271/870403

 

Yep, as i note, he had other long posts. Could not find his re mentioning oil temp after cold starts. BUT IMO the1993 Owner's Manual before the OLM was used in the Vette, proves that point. Don't even know if ZAP is as relevant today.

 

Yeah, I hesitated to mention that, but the amount of the zinc dialkyldithiophosphate (ZDP or ZDDP or whatever) or equivalent (zinc, phosphorous compound) is much lower in modern oils. Still present, and actually in Mobil 1 Supercar 0W-40 its not real low. The algorithm does depend on assumptions about the oil and its additive package.

 

Yep, but in the SAE Doc link you provided, unrelated to ZAP it says:
"LOW-TEMPERATURE, SHORT-TRIP SERVICE -
The other major mode of oil degradation occurs in low-temperature, short-trip service. Corrosion and wear are often associated with this type of operation An increase in wear can be caused by a reduction in viscosity (as a consequence of fuel contaminating the oil on cold starts) to the point that there is a shift from hydrodynamic to boundary lubrication. The oil can also be contaminated with acidic products due to partial fuel combustion (4). These acids may attack engine metal at the site of condensation, or they may so reduce the basicity of the oil that the oil is no longer able to protect the metal surfaces throughout the oil system. In either case, metal residues are readily found in the oil once the oil is no longer sufficiently alkaline (11). During low temperature operation, oil is also contaminated by water, a normal combustion product. If oil temperatures do not exceed 100 C, water may remain in the oil for extended periods, causing rust accumulation on non-rubbing surfaces. Water and acids also may react directly with antiwear and antioxidant additives, reducing their effectiveness."

Thought this Figure was interesting. It defines perhaps why the 1993 Vette Owner's Manual suggests max 3000 miles for oil changes in Short-Trip Operation! Appendix C has some discussion of time but introduces new factors not important to introduce here.

Figure from the 2018 SAE Paper by Schwartz and Smolenski.
It shows suggested "miles" to change oil if short trips where oil did not get above 125 F, is ~1500 to ~2500 miles (note data in km)
It's no coincidence that is how Jiffy Lub and many dealers justified:
After an Oil Change putting a sticker inside your car on the upper windshield that said: "You'all come back" at a date 3 months after and an OD reading 3000 miles added to the OD reading." There was data to back that up.
And "they" did not know how you drove nor most owner's did not know what sever servcie was! Certainly Garage Queens that mostly just drove to the local store 3 miles away is severe service! Starting the engine and letting it idle for a short time to "let the oil circulate' is WORSE!

 

Yes, that's the point! It discusses much more than just the Zinc Phosphorous additive.

 

I got my car with 5 miles on odometer but it was built 5 months prior to my purchase sitting in showroom. Dash said 60% oil life when I bought it. Now a month and 500 miles later says 50% life. At this rate monitor will tell me change oil at 6 months and 2000 miles. What should I do?

 

Remember the oil:minder is also time based, it will go to zero if you don’t drive it at all for a year. I would change it when I got down to 5% , reset it and get on with your life. it is chrvy's opinion that even if the oil sits and the car is not being used, it is degrading. That’s up for debate, but nevertheless…

 

60% life remaining is about right for 5 months. However the dealer should have reset it to 100% when the car was first put into service (sold to you). I thought I read something somewhere that the fluid life monitors weren't supposed to start counting down until the car has something like 50 miles on it but I could be mistaken.

You should call your dealer and ask them what to do. They may tell you to change the oil 1 year from when it was first put into service if you don't put many miles on your car (you'll need to reset the monitor or it will nag you when you get close to zero). Or they may tell you to bring it in when it gets down to zero and they might give you a complimentary change (don't let them use your first free oil change for this). I'd also check when your warranty expires and make sure it didn't start 5 months before you bought the car. The warranty is supposed to start when the car is first put into service but some dealers don't do the process right or something.

 

Spoke to ciocca service manager who emphatically told me to follow the dash recommendation as the oil degrades over time even from new.

Imo they should have provided me with fresh oil since car was sitting on their lot so long but I don't really care since I was always going to pay to change oil at 1500 miles anyway then use the free service at 7500. No biggie