230-240

 

I think the story with VI improvers is about tradeoffs.
With a big viscosity spread you get oil that's thinner when cold (good) and thins out less when hot (good).
With the VI improvers needed for big spreads like 0W-40 there's the possibility of the VI improvers breaking down after high miles, and giving you an oil that's thinner than desired (bad). And the oil costs more.

GM has tested the 0W-40 for both track and street use, and are satisfied that it performs well in both regimes when changed according to the OLI. Mobil agrees. My own testing shows that the 0W-40 holds viscosity better than 5W-30 if you change when the OLI tells you, and probably somewhat longer.
If I were trying to set some sort of "most miles between oil changes" record, and couldn't do lab testing every thousand miles or so, then I'd go with a 5W-30.
For normal street driving with normal oil change intervals, I think the 0W-40 is probably a bit better. But the chances of it making a big difference in your engine life, are slim.
Track rats are a different animal.

 

correct, but it's important to remember that the viscosity modifiers are not magic. They don't make it a thin oil when cold and a thick oil when hot.
They modify one specific viscosity to make it ACT like a thin oil when cold and a thick oil when hot.
There's a big difference.
And the bigger spread you want to get the more you need to push that reality of what oil is and "fake it" per say.

Bottom line, a straight 40 weight on the track would be objectively better than a 0w-40
Obviously you can't start that engine up when it's 20 degrees outside, that's the drawback.

 

The newly recommended oil held up better than the old one? Color me shocked.

 

ha ha
yes, in a completely uncontrolled experiment with multiple variables not monitored in any way or accounted for
case closed

 

Now you're getting why blanketly saying the 0w40 is not ideal doesn't hold water either. There isn't a "best." It's all trade-offs.

Deviating from the GM recommendation is fine, just know why.

 

you keep saying I'm making a blanket statement when that never happened.
I specifically said for street only use, which is true.

 

If you think "street use" encompasses a narrow set of use, then we aren't speaking the same language.

 

if you think different types of "street use" require different oils then. . . . . .well. . . . nevermind
I'll just go with that, sounds reasonable as long as you don't think about it.

 

Someone in -20 winters should choose the same oil as someone with 100 degree summers?
What about someone that idles for minutes at a time? Should they keep the same OCI?

It's all street use!

 

ever see those charts for the temp range that multi viscosity oils are spec'd within?
yep, those are a thing for street use
5w-30 isn't good at -20 and it's spec'd as such

 

Thanks

/ end thread

 

There’s a reasonably good understanding of VI improvers in most of the posts in this thread, but a few points still may be getting missed. So let me go back and make a few points about dino oil and see if that reduces or adds to confusion. I’ll start with three grades of dino oil that do not have any VI improvers. They are single grade 5 weight, single grade 30 weight, and multi grade 5W30. But there’s a catch with 5W30 dino oil. You can only make it from one source of crude oil in the world, namely, crude from a small area in Pennsylvania. That’s why the two companies that cornered that source, Pennzoil and Quaker State were the only two companies in the world who could make 5W30 until VI improvers came along in the 1960’s and 70’s.

To make lube base stock from dino oil, you first remove the wax and most of the aromatics, both of which are easy to do. Trouble is, that still leaves you with naphthenes (which there is no good way to remove) and some residual aromatics, both of which are poor lubes because they thin our rapidly as temperature increases. But Pennsylvania grade crude has essentially no aromatics or naphthenes, so its lube base stock is essentially pure isoparaffin, which does not thin out nearly as fast with temperature.

With most liquids, viscosity drops as temperature increases, and the relationship if you plot it on semi-log graph paper is more or less a straight line. But since the Pennsylvania grade base oil didn’t thin out with temperature as fast as other base oils, its temperature/viscosity line had a fundamentally flatter slope. No tricks involved, that’s just the way it behaved. When they were setting up standards for multi grade oil from Pennsylvania, they arbitrarily set 32F as the comparison point for the W number and 212F for the upper number. Thus at 32F, 5W30 behaves and lubricates like a 5 weight single grade, while at 212F, it does so like a 30 weight single grade. But if you think about the way a straight line plot looks, the advantage for the multi grade keeps increasing both on the cold end below 32F and the hot end above 212F. That’s because the multi grade slope is flatter, so if you went to 0F, it wouldn’t just be the same as the 5 weight, it would be considerably less thick. Similarly, if you went to 250F, it wouldn’t just be the same as the 30 weight, it would be considerably less thin.

OK, so now enter VI improvers. Remember the octopus analogy. At low temps, the molecule looks like an octopus with its tentacles wrapped tightly around its body, so it has essentially no impact on viscosity. However, at high temps, the tentacles unfurl, so that they interfere with flow, thus increasing viscosity. So if you want to make 5W30 from dino oil that is not from Pennsylvania, where do you start? You start with 5 weight single grade and add VI improvers. Trouble is, there are two drawbacks. First the octopus tentacles can shear off, and as they do, the viscosity of the upper number in your multi grade oil drops, and once fully degraded, you are left with single grade 5 weight oil. VI improvers are getting better, so that’s not nearly the issue it was in the early days of VI improvers, but it’s still something to be concerned about. The other issue is that at some point (namely, the point at which the octopus tentacles are fully extended) the slope of the vis/temp line reverts to base oil slope. Thus you lose the continuing advantage discussed at the end of the prior paragraph. So unless your driving habits need the flatter slope from the VI improvers, you are better off without them.

Now on to synthetics. The big advantage of a full synthetic is that like the Pennsylvania grade crude, synthetic base oil gives you about a 25 spread without VI improvers. So for example, you could make 5W30 or 15W40 from synthetic base without VI improvers. But 0W40 starts out as 0W25 base oil, and you get the last 15 spread from VI improver (degrading back to 0W25 as VI improver shears). Similarly, 15W50 starts out as 15W40 and you get the last 10 spread from VI improver (degrading back to 15W40 as VI improver shears). In other regards, there are the same characteristics as the prior paragraph, so like the prior paragraph, if you don’t need the spread, why accept the higher cost and the penalties of VI improver. I don’t track my car and I live in Texas, so I don’t need them, and have always run 5W30 in my cars, which have included 3 base engine Vettes, a CTSV coupe with supercharged Vette engine, and a turbocharged BMW 550.

Cheers. If you’ve stuck with it this long, I hope you found some of it useful, or at least interesting. And by the way, all the Pennsylvania grade crude is long gone, so if you want the advantage of flatter vis/temp slope without VI improvers (up to 25 spread) you are stuck with synthetics.

 

Great info from LarryThe Oil guy. You have a gift for ‘splaining oil. Much appreciated Larry!

 

good write up
sounds like big spread oils with lots of VI's aren't as good unless you absolutely need them, like for the track but not the street

 

Mostly correct, but it’s not actually the track that needs a wide spread. At the track, you are hot all the time. So if you have a car that’s strictly for track use, there’s no obvious reason to go much, if any, over a 25 spread synthetic. Why risk VI improver shear on the hot end when the hot end is where you mostly run? The wide spread is only a highly desirable characteristic for a car that runs both street and track.

There’s also a minor and somewhat picky nomenclature issue in your post. VI does not stand for viscosity improver, it stands for viscosity index, which is the standard technical measure of how rapidly the viscosity changes with temperature. So when discussing the additive, the correct term is VI improver. Orrrrr, the layman’s term viscosity modifier that you used in some of your earlier posts would work too. Just avoid using VI by itself when referring to the additive.

Finally, thanks to you and Need for Speed for the kind comments (and Gearhead Jim for his thumbs up).

 

Mobil1 5w50 super car for the new 2025 1064 hp ZR1

 

I don't track anymore so not sure what the newer cars do but besides warming up when getting ready, aren't you almost always at temp and top end at the track? So wouldn't I want a narrow spread but long lasting viscosity oil to protect me when I am tracking since I am at temp for most of the day? On the street it could be different since I would start it up in the morning from early spring to late fall and it needs to protect the engine from startup to stop to startup to stop, etc, etc?

 

that’s kinda what he said?

 

Two different types of "street use":

1. A guy lives in Minnesota where winter temps can easily hit -30'F. He lives one mile from work, car parked outside. He starts the car each morning, drives 1 mile to work and shuts down. Car sits outside all day, daily high is 0'F. At quitting time, he fires up his car and drives 1 mile home.

2. A guy lives in the southwest where daily highs can easily hit 120'F. He lives 30 miles from work and has a heavy foot. Gets stuck in traffic for 30 minutes in the heat, engine gets toward upper limit temp, but often uses full power/full rpm to pass slowpokes on a two lane road for 20 miles.

Both guys are "street use", but I don't think a single oil would be best for both engines, except possibly something like a 0W-40 and you take appropriate precautions regarding oil life.

BTW, example #1 was my Dad many years ago, and example #2 was me many years ago.
Our 3 previous Corvettes were Z51 automatics, no engine oil cooler. Even moderately aggressive canyon driving on a hot day would get the oil temp up to 270'F. Our C7 Z51 has better coolers.