pastorbrad

Probably very simple except for folk like me. Car (14 base C7) hadn't been driven since late August due to medical issues. Finally got it out this week for perhaps 35 miles total. At the start the oil indicator read 68%, at the end 20%. The oil was last changed in July 2018. Is there a time factor in these readings or...? Just curious why the large drop in a short stretch. thanks

Bill Dearborn

Yes, GM added a time factor in the C7 OLM configuration. Prior to the C7 the oil change rules had been to follow the OLM or change once per year which ever came more often. Must be that a lot of people weren't paying attention to the once per year part of the rules so it is now included in the OLM algorithm so the oil life will decrease with the car just sitting in the garage without the engine running.

Bill

Bavaria

It's full synthetic! I change after 10k miles on all our cars, regardless of how long it takes to get there (sometimes close to two years). Never had a problem, with two of our cars - bought new 2007 and 2010 - hitting respectively 94k and 72k miles. Also, neither uses oil between changes, nor has the C7 after almost 8k miles since last change in Dec. 2017.

Skid Row Joe

The time factor is for 1-year. Which is 100% arbitrary on oil life, even if it's never been used.

pastorbrad

Thanks! Makes perfect sense.

Vetteman Jack

Yeah, the fact that the OLM also takes into account a time factor is quite the change from previous generations.

Obe1 GS

Tadge had this to say on the topic:

Tadge answered:
Great question, Patman. This is one I bet a lot of people are curious about. Obviously, maintaining the quantity and quality of the oil in your engine is paramount for long term durability and performance. There are many things that work to deteriorate the quality of your oil and we make the oil life monitor (OLM) as accurate as we can to predict oil life in consideration of all of those variables. The simplest variable is the length of time the oil is in the engine. As you surmise in your question, this is an important variable. Oil ages sitting inside your engine differently than in a sealed container. It is exposed to more oxygen, humidity, various sealants and a variety of metals in your engine. Also, time causes things you might not expect to have an influence such as oxidation of your oil filter. To account for this variable there is an OLM clock that begins a year-long count down every time it is reset. So that is the best you can do…. Change your oil and filter once a year.

You also surmise that mileage should be a factor. We could use mileage, but it is more accurate to actually count combustion events. Every time the fuel and oxygen ignite in the cylinder a tiny quantity of contaminants slip past the piston rings and gets into your oil. Over time, these contaminants build up and hurt the oil’s lubrication capability. You can see this effect as the oil darkens over time.

OK, but all combustion events are not created equal, right? There is a big difference between cruising down the highway under light load and tearing it up on the track. On the highway, the coolant and oil stay relatively cool and there is little stress on the engine. On the track, when oil temps are high, oil molecules actually fracture and that hurts lubrication performance. We handle that buy adding a multiplier that compounds the number of combustion events that occur at higher temperatures. The hotter the oil, the more heavily we weight those events and the shorter the life prediction. On the other hand, running your engine at low temperatures stresses the oil as well. So we also have a multiplier that over-weights combustion events when it is cold. People who drive in cold climates and take relatively short trips are going to find they have shorter oil life. The display in the cluster is regularly updated to the lesser of the time-based or use-based metrics.

Lastly, to get to the first part of your question: No, the dry-sump cars do not use the same algorithm as the wet-sump. As you theorize, having more oil dilutes contamination and distributes thermal stress across a greater quantity of oil. Those are both true and, even though you get some life extension, it is not in direct proportion to the total quantity of oil in the system. Why not? Well the time-based degradation is very insensitive to oil quantity. And don’t forget the oil in dry-sump cars get more exposure to air and system metals (and other materials) because of the tank and plumbing. The extra oil quantity helps roughly in proportion to volume for contamination, but not quite in proportion when it comes to thermal stress. Since dry-sump cars are more likely to see track duty or hard street driving we tend to be a little conservative to make sure oil is changed before it falls below a performance level needed to assure perfect operation of your engine. Bottom line is that, depending how you use your car, the dry sump will have at least the life of wet sump, and probably somewhat more.

JerryU

Good post had not seen that one. He briefly mentioned what "Bob The Oil Guy" who worked with the folks at GM that developed the OLM system emphasized about changing with time. Tadge said as you quote:
"On the other hand, running your engine at low temperatures stresses the oil as well. So we also have a multiplier that over-weights combustion events when it is cold. People who drive in cold climates and take relatively short trips are going to find they have shorter oil life. The display in the cluster is regularly updated to the lesser of the time-based or use-based metrics."

Perhaps the best actual example of what "Bob" said is considered by the OLM algorithm is what was defined in my 1993 Vette Owner's Manual before there was an OLM and only measure was miles driven and time. It stated:
Change oil at 7500 miles OR one year, whichever occurs first.
BUT if most drives are under 4 miles change at 3500 miles OR 3 months whichever occurs first!

Blowby on cold starts is a major oil contamination issue. IF you drive long enough after a cold start to get the oil hot, most water, a main product of combustion, will evaporate and not be available to form with contaminants like sulfur to produce sulfuric acid and other bad compounds. The OLM takes that into consideration and if the oil does not get hot enough, long enough after a cold start it will shorten the time for change to less than one year. That is what the '93 Owner's Manual was trying to avoid.

Worse thing for a car engine is backing it out of the garage to access something and driving it right back! For my Street Rod, that mostly is just driven to Car Shows, if I have to pull it out of the garage to get a ladder, I have a 20 mile run to get the oil hot (and have fun!) Those large forged pistons in the 502 cid engine contract enough when cold, it sounds like a sewing machine for about a minute. Lots of water and fuel from the rich mixture in the 850 Holley when the choke is set! The C7 pistons are somewhat smaller and cast aluminum that contracts less but it's still an issue.

Red86Cfour

Ok, but what do the bearings and other wear surfaces look like after nine and seven oil changes, respectively, on your two cars? Just because they still run isn't a good metric of oil change interval time. Send that oil to blackstone labs, or some other analysis lab, and see what they say about the oil that comes out.

fdxpilot

have fun with a power train warranty claim, since GM requires at least once a year, no matter what mileage.

Bavaria

In almost 60 years of driving over two dozen cars, from a '58 Volkswagen to the C7, I've never had a power train issue.

mcoomer

I'm still kind of skeptical of the OLM calculations. I'm all for regular oil changes, based on time and mileage, but I wonder at Chevy's math. I replaced the oil in my Z in preparation for a trip down the coast in December. Ended up doing just over 3,000 miles and OLM was in the 30's after the trip. I think they're really conservative with their calculations.

JerryU

^^^^
It's not math it's a complex algorithm based on a lot of inputs like: how hot the oil gets and how long after a cold start; like the engine revolutions as mileage is foolish with highway being much better than around town, and a number of others.

Yep you can go by what was used when all the input that was able to be analyzed was the Odometer reading and time. And if mostly short trips change oil at 3500 miles or 3 months WHICHEVER COMES FORST! That is what was stated in my 1993 Vette Owner's Manual before there was an OLM to account for the biggest contamination, cold starts.

The oil doesn't wear out in a year but that sulfuric acid etc formed after a cold start when the oil doesn't get hot enough to evaporate most of the water that blows past the cold piston and rings.

Yep, silly today when they can crunch other input data from the many sensors available.

But your car do as you wish!

Patman

That doesn't guarantee it will never happen. Why roll the dice when it's not that expensive to do an oil change every year? Seems foolish.

pastorbrad

When the car (2014) has sat from November until now - the OLM reading went from 38 to 5 on startup today because of the time factor. I would call that being unduly alarmed. The car will be driven today for long enough to warm up fully, etc,, so there's still some slack before it absolutely HAS to have oil changed again!

JerryU

^^
Hmm, even that way since my 1st Vette, a 198 but you had to mark a calendar. It said change oil at 1 year max!

In fact my ‘93 9before the OLM) said if most drives are less than 4 miles change oil at 3000 miles OR 3 months WHICHEVER OCCURS 1ST! The worse thing for oil is cold starting, with the rich mixture and cold pistons having lots of blowby AND not getting the oil hot enough long enough to burn off most of the water (product of combustion!). That forms with things like sulfur in gas and oil producing things like sulfuric acid that eats at metals! You’re changing oil for that reason NOT that it wears out

It typically will just say change one year from the last fill so don’t fret about the reduction when not using! It’s just like looking at a calendar!

Gearhead Jim

My mother smoked a pack a day for her entire adult life, and never noticed any serious effects.
But I choose not to smoke.

Bavaria

My cars don't smoke either.

Skid Row Joe

I'm not sure if smoking one's entire adult life, wouldn't produce serious effects. Depending on defining those effects.

The best advise when it comes to C7 engine oil, and oil filter change intervals, is to follow the particular year and model owners manual instruction.

Skid Row Joe

It's really up to the individual owner's discretion. There's example after example of extended time and mileage synthetic oil usage without any noticeable effects. You'll never go wrong with following your C7's owners manual advise. Glad you're medically back and up to driving your C7! 👍