^ You seem to be lost in the numbers.
All else equal, it is a downgrade to choose a winter rating lower than you need, and settle for a lower base oil viscosity to get there.
You don't seem to be processing my explanation.
If we are looking at two xW-40's and one has 11.17% VII and the other has 7.68%, which one do you think has the lighter base oil blend? Furthermore, which one is going to be more inclined to suffer mechanical shear?
As I said, all else is basically never equal, I actually provided some examples of how this plays out in practice. They just use cheaper bases.
I think the problem here is the assumption that the oils will be blended like this, with the narrower the range, the less VII and the heavier the base oil blend. In this example, the 0W-40 has around a 5cSt base oil viscosity and 12% VII, while the 10W-40 has around a 7cSt base oil viscosity and 7% VII.
Whereas, the manufacturers can instead get away with doing something that looks more like this, where it is 5cSt and 9.8% VII:
That's why I brought up Gokhan's spreadsheet, because he does have VII estimates for a few 10W-40's and they aren't super heavy base oil with almost no VII like one might hope to see.
Again, going back to that same spreadsheet and Gokhan's estimates, if we look at two Mobil oils:
M1 HM 10W-40:
Base oil viscosity: 7.36cSt
VII content: 11.17%
M1 FS 0W-40 (Visom):
Base oil viscosity: 7.42cSt
VII content: 7.80%
So, with these two Mobil examples, the 0W-40 has less VII, and a heavier base oil, than the 10W-40.
So, back to my point, you can't just assume that a 10W-40 has a heavier base oil blend and less VII.