Yeah, it's more about the natural viscosity index of the base oil that seems to be the determining factor, and even then the finished lubricant need not be marketed as a multi-weight as has been reported with Amsoil's single wts. (a 30wt behaves more like a 10w-30 for example purposes only).
VII's need be utilized for oil grades whose finished grade requires for greater fluidity over a designed temperature range that the base oil of lesser natural viscosity index cannot accomplish, and because the VII's themselves are of much higher viscosity than the base oil if not even a solid by themselves (so people have said around here), requires the base oil viscosity to be even lower in value so as to reach the target viscosities.
A natural tendency of VII's in shear stress is to align themselves in the region so as to cause a momentarily thinning of viscosity with values approaching the natural viscosity of the base oil itself, (remember the base oil is now thinner to account for VII thickening the final formulation - other additive effects aside). This shearing of a momentary sort helps to reduce viscous losses though presents thinner film serving as a buffer between the moving parts, making for a greater overall variance in lubrication regimens. This is where anti-wear additives pick-up the slack, where of course they can be effective (EP films don't work on substrates whose structural strength is less than that, for the base structure will give and the materials will be damaged/lost. ZDDP films on elemental aluminum for example). Dependent upon VII quality and the severity of shear/service, the VII's may be permanently sheared, lowering the viscosity value of the finished lubricant in service before/while oxidation begins to lead to thickening. The Overall thinning might be what is in mind for energy conservation, I'm still not fully engrossed in lubrication realities as yet...which reminds me - take this all with a grain of salt!
Off my box...
