[keep_hope_alive]

Originally Posted by ctrlz

Not sure keep_hope was around when I previously posted this.
If you download this guy's thesis (it is free), page 11 describes the GM algorithm in detail. Oil geeks rejoice!
https://etda.libraries.psu.edu/catalog/9507

awesome!

I hadn't seen it.

[keep_hope_alive]

Originally Posted by ctrlz

Not sure keep_hope was around when I previously posted this.
If you download this guy's thesis (it is free), page 11 describes the GM algorithm in detail. Oil geeks rejoice!
https://etda.libraries.psu.edu/catalog/9507

For those TL : DR, I pumped the GM portion of that paper into ChatGPT for a summary:

The passage you provided discusses a study conducted by General Motors (GM) to investigate the effects of trip length and oil type on engine and engine oil degradation. The study aimed to compare the degradation of synthetic oil versus mineral oil and the impact of short-trip service versus long-trip freeway service on engine oil.

The study used a high-performance sport vehicle with a port-fuel injected 5.7L engine. Different driving conditions were tested during three oil-change intervals. Here is a summary of the tests performed:

1. First oil-change interval:
- Vehicle driven for 10,000 miles of free service using synthetic oil.
- Followed by four weeks of low-temperature short-trip, cold-start service.

2. Second oil-change interval:
- Vehicle driven for four months in low-temperature short-trip service using the same synthetic oil.
- Followed by 100 miles of long-trip service.

3. Third oil-change interval:
- Vehicle driven for 10,000 miles of freeway service using mineral oil.
- Followed by four weeks of short-trip service.

Throughout the tests, oil samples were taken at specific intervals and analyzed for physical and chemical properties. The study observed several changes in the engine oil based on different service conditions:

1. Water Contamination:
- Short-trip service in winter resulted in high water contamination levels, even with synthetic oil.
- Short-trip service in summer showed no water contamination.
- Water was removed from the engine oil within 20 miles during long-trip service.

2. Fuel Contamination:
- Fuel entered the engine oil during long-trip service.
- Short-trip service increased fuel concentration in the oil, especially in low-temperature conditions.
- No apparent loss of vehicle performance was reported.

3. Viscosity:
- Fuel entering the engine oil during short-trip service caused a decrease in viscosity.
- Increased oil temperature due to warmer weather or longer trips resulted in a rise in viscosity as fuel evaporated.

4. Antioxidant Protection:
- Short-trip winter testing showed rapid loss of oxidative stability in the oil.
- Warmer weather or longer trips decreased fuel concentration and increased oxidative stability.
- Synthetic oil maintained a significant amount of antioxidant protection, while mineral oil showed marginal antioxidant protection.

5. Iron Content:
- Short-trip service led to a higher rate of iron accumulation compared to long-trip service.
- Water presence in the engine oil suggested corrosion as the primary mechanism for iron entry.

6. TBN (Total Base Number):
- TBN dropped rapidly during short-trip service but was partly reversible.
- Short trips caused a similar reduction in TBN as longer trips.

7. TAN (Total Acid Number):
- Synthetic oil showed little change in oil acidity during long-trip testing.
- Mineral oil exhibited a slow rise in acidity during long-trip testing and a drop during short-trip testing.

Based on the results, GM suggested that vehicles using a borderline SG-quality mineral oil could be driven for 10,000 miles under freeway conditions without significant oil degradation or engine damage. For synthetic oil, the recommended interval exceeded 12,000 miles. However, during short-trip service, rapid oil degradation occurred, and an oil-change interval of 3,000 miles was recommended.

The passage also mentions factors and conditions that can either shorten or lengthen the oil change interval. Factors that shorten the interval include short-trip driving in winters, road dust, high-mileage engines, diesel engines, flex fuels, turbo-charged engines, high oil consumption, hot running conditions, and heavy loads. Factors that lengthen the interval include synthetic lubricants, high capture efficiency oil filters, predominately highway

.

[ZPirate]

Originally Posted by Fun

Seems too soon to change. Wondering if they didn’t reset it to 100% at last change. Think I’ll get it changed.Thoughts anyone?

Mine is in a similar situation. My car has approximately 3,800 miles on it. Oil was last changed at 1,500 miles by the dealership using the one free oil change in August. I suspect that the dealership didn't reset the oil monitor as I initially changed the oil at 500 miles since the car only has 2,300 miles on it since the last oil change. I am planning to change the oil again at 5,000 miles or one year since the last change whichever occurs first.

[Fun]

Originally Posted by ctrlz

yes, indeed.
But I'm guessing OP would not be asking he put some hard miles on the car.
personally I think his theory is spot on.

I treat my car like a baby. It’s a 2020 that I’ve had since October of 2019. It just turned 19,500 miles. I’m averaging 5,400 miles per year.
Thanks everyone

[Matt1LE]

That's right for the V6. GM changed the way it's calculated since too many engines were grenadeing due to premature timing chain failures.



watch this video:

[ember1205]

Originally Posted by ctrlz

As I said above, mine drops 2% per week without fail.
I change it at about 4%.
The math doesn't work for 11% and 7 months with only 3800 miles

Why not? 7 months is 30 weeks. 2% per week would be 60% based on time alone which would result in 40% remaining.

[ctrlz]

Originally Posted by ember1205

Why not? 7 months is 30 weeks. 2% per week would be 60% based on time alone which would result in 40% remaining.

Agree it should be in the 40% range, not 11%

[ember1205]

Originally Posted by ctrlz

Agree it should be in the 40% range, not 11%

But that's time -only-, though. What about the 3800 miles driven? That in itself also represents a 40% reduction based purely on mileage.

I'm not surprised at the 11% reading based on these data points, especially if the car sat for a number of days at a time without being driven.

[ctrlz]

Originally Posted by ember1205

But that's time -only-, though.

My car seems to care only about time, or at least my driving style/miles don't seem to impact the algorithm.
This morning oil life read 66%. I changed oil 16 weeks ago.
100 - (16x2) = 68.
Ok, so a 2% difference.

[ember1205]

Originally Posted by ctrlz

My car seems to care only about time, or at least my driving style/miles don't seem to impact the algorithm.
This morning oil life read 66%. I changed oil 16 weeks ago.
100 - (16x2) = 68.
Ok, so a 2% difference.

I know that time and distance are likely the top two contributors to the calculation but I don't know how they might "interact". For example, would one apply a 60% reduction for 30 weeks of time and the apply a 38% deduction from that based on the percentage of miles compared to 10K? That would bring you down to 25% (and you could reduce by 38% for miles and then 60% for time and still be at the same number - thank you, math).

Where does the additional 14% of total life come from? Is the ECM seeing high rev's? Hard acceleration? What about detecting moisture levels? What other items is the ECM capable of factoring into the number and how does that relate for this specific situation?

As I said... the 11% doesn't really surprise me, although I can't state with any sort of certainty how it got there.

[keep_hope_alive]

"Using the engine oil temperature value and figure 1-1, corresponding oil temperature penalty factor (TPF) is obtained and using the engine oil contaminant value and
figure 1-2, corresponding oil contamination penalty factor (CPF) is obtained. The penalty factors increase the measured engine revolutions value to compensate for engine operating conditions that tend to degrade the engine oil. Periodically during each period of vehicle operation, an effective engine revolutions (EER) value is determined in relation to the product of measured engine revolutions (MER), TPF and CPF. The EER value is subtracted from the stored maximum number of engine revolutions resulting in a remaining allowed revolutions (RAR) value. Each time the EER value is calculated and subtracted from the RAR value, a new RAR value is stored in memory, replacing the old value. Equations 1.4 to 1.7 give a list of calculations carried out in order during each period of vehicle operation. After each period of vehicle operation, the stored RAR value is compared with a predetermined threshold number of engine revolutions (TNR)
value indicative of the useful life of the engine oil and once RAR value goes below the threshold value, the system actuates an indicator advising the operator to change the engine oil. "

[ember1205]

Originally Posted by keep_hope_alive

"Using the engine oil temperature value and figure 1-1, corresponding oil temperature penalty factor (TPF) is obtained and using the engine oil contaminant value and
figure 1-2, corresponding oil contamination penalty factor (CPF) is obtained. The penalty factors increase the measured engine revolutions value to compensate for engine operating conditions that tend to degrade the engine oil. Periodically during each period of vehicle operation, an effective engine revolutions (EER) value is determined in relation to the product of measured engine revolutions (MER), TPF and CPF. The EER value is subtracted from the stored maximum number of engine revolutions resulting in a remaining allowed revolutions (RAR) value. Each time the EER value is calculated and subtracted from the RAR value, a new RAR value is stored in memory, replacing the old value. Equations 1.4 to 1.7 give a list of calculations carried out in order during each period of vehicle operation. After each period of vehicle operation, the stored RAR value is compared with a predetermined threshold number of engine revolutions (TNR)
value indicative of the useful life of the engine oil and once RAR value goes below the threshold value, the system actuates an indicator advising the operator to change the engine oil. "

So, it's all a mystery.

I mean, I get it... One COULD make a reasonable attempt at calculating it all out. But the reality is the ECM looks at a number of factors and figures it out for you.

[esetter86]

Found this thread looking for information on how GM calculates the oil life, was wondering if my situation was unique, apparently not.

2021 LT1 6MT, took delivery in July of 21. Currently at 2,600 miles and oil life is at 20%. Local GM dealer did a Mobile 1 oil change last summer at roughly 1,500 miles. Kinda annoying to be needed my third oil change already!

[cellsafemode]

if you use full synthetic and don't track your car, you should be fine for well over 5000 miles or a year between changes. I'd let the oil life meter hit 0 if you were under that and not talking about the very first oil change.