Can't speak from a gasoline perspective, but I know from my commercial UOA's that 10w30 shows less viscosity change than 15w40 in my diesels. And to the OP's point, 15w40 uses more VI than 10w30. That is comparing two of the same brand name products, both syn blends. I have not really compared 5w30 to 10w30 on that level.