shear stable has two facets...temporary shear, which is the non newtonian bit when thy thin at high shear rates, to an apparent new viscosity, but recover when the shear rate goes back down...the reason for HTHS being introduced into J300.
The second is permanent shear, which is the VII molecules being torn up, reducing both the KV AND the HTHS, permanently...that's the "shear stability" that is typically referred to in the search for better VIIs.
For the purpose of this discussion, lets assume that any additive that changes the viscosity behavior of a fluid is a VII.
OK, to avoid my apparent confusion, I'll defer to SAE J300
Many commercial engine oils contain polymeric additives for a variety of purposes, one of the most important of which is viscosity modification. Specifically, the use of such additives in creating multigrade oils is commonplace. However, oils containing a significant polymeric additive concentration, whether for viscosity modification or another lubricant function, are generally characterized by having a non-Newtonian, “shear thinning” viscosity (i.e., a viscosity which decreases with increasing shear rate).
Most oils will meet the viscosity requirements of at least one of the W grades. Nevertheless, consistent with historic practice, any Newtonian oil may be labeled as a single-grade oil (either with or without a W). Oils which are formulated with polymeric viscosity index improvers for the purpose of making them multiviscosity-grade products are non-Newtonian and must be labeled with the appropriate multiviscosity grade (both W and high-temperature grade).
they are saying that to carry the straight weight label, they must
be newtonian, e.g. the viscosity at 150C is the same when running by gravity through a drop tube as it is in a rotary viscometer at 10^6 shear rate.
PAO increases the VI of an oil , but does not display the behaviours of those chemicals that are expressly employed to provide VII...they display characteristics in mixing (and they are blended in the single digit fractions like tenths, quarter and halves), not added in small percentages to affect great changes (like doubling the KV100 with a couple of percent), which is the role of VII polymers.