[aporigine]

I had a long answer typed out but I just blanked it. I've been looking up viscosity index improver chemistry, expecting to find a single class of molecules being the major article of commerce.
It ain't so.
VIIs in general are plastics. Two examples I've found are ethylene propylene polymer, and polyalkyl methacrylate. (Polymethyl methacrylate is Lucite/Perspex). Those two families alone have very different responses to thermal and mechanical stress. Early VIIs had short service lives, leading to definite degradation of HTHS (high temp, high shear) viscosity after a few thousand miles, and some even produced a waxy residue on bearing surfaces.
Bottom line is that I need to amend my "old wisdom" in re VIIs. There seem to be some that have excellent stability and benign behavior once degraded. Finding out what's what is made a bit of an adventure because oil formulators/sellers treat the hard info as proprietary. I expect that a basic group 2 oil sourced from certain far East producers will contain traditional, flawed but dirt-cheap VIIs. Caveat ecomoddor!!

I would expect that a premium oil using a group 4/5 base stock also uses advanced VIIs. I don't have info on whether or not this is so. In a wide-spectrum multigrade, VII makes up typically ten per cent by weight of the finished oil. Concentrated VII is not as good a lubricant as base stock, so as a mileage-obsessed traffic barnacle I will seek oils that use less of them. That's what choosing certain products using a blend of PAO and ester gain me. A longer recommended change interval is pure bonus.
cheers apo

<edit> I have been finding contradictory info on M1's base stock:
1) It has always been and continues to be based on group 4 PAO.
2) It was PAO but is now largely group 3 mineral-sourced.
If anyone has the definitive word w/relevant link ... thanks in advance!!