We Are Obsessed But Does It Really Matter


$100 Site Donor 2021
Apr 28, 2008
Ontario, Canada
I do have a question… The “Winter” number is actually above the ambient temperature in almost all instances and you really need to know where the viscosity at startup is. 5w20 has less VII than 0w20. Are VII only targeting down to 104 deg. F and 5w20 actually thinner at ambient start temperature due to much less VII? Of course the base oil type would get into this as how much that base oil gives you a better VI.
Viscosity Index Improvers, as the name states, improve the Viscosity Index, which of course is the spread between the 40C and 100C visc. VII is how you get oils like TGMO 0W-20 with a VI over 200 for example, you are limited to around 140-150 (though Dave has made an exception to that) when limited by the VI of the base oils.

The effect of VII decreases with temperature, they "coil up" and contribute less and less as the temperature is reduced.

Widman has a VI calc on his site:

I'd recommend playing around with it a bit to get a "feel" for how this works.

For example, let's choose an 8cSt (@100C) oil.
If it's 45cSt @ 40C, the VI is 151
If it's 40cSt @ 40C, the VI is 178
If it's 35cSt @ 40C, the VI is 212

Typically, the 0W-xx oils have higher VI's. That's not always the case, but typically. Which means they'll also typically be thinner at starting temperatures.
Jul 12, 2012
Caldwell Idaho
If oil flow were the only cure for the heat that is generated, then why are air-cooled motorcycle engines recommend to use heavier oils like 20W-50 as opposed to something like a 0W-16?

My point is that viscosity itself is a way to protect when increased heat is a factor.
The 20 leaks out of the bearings too fast to pick up the heat.
May 7, 2020
Ames, IA

Though it depends on where you live. Here, rust will kill your car before the engine dies from lubrication.

I managed a fleet where drivers were responsible for maintenance. Some were great, some reset OLM’s without a change, and some did it haphazardly.

We never had a lubrication issue, outside of turbo seals on some Ecoboost trucks.

We worry too much, and that’s ok.

I don’t care about sports or play golf. Cars are my thing.