My take on the difference: The viscosity modifiers make it behave as a 30 weight at the bearing crank interface under load and shear. But, the ring to cylinder is a different situation, as there is no load, just a sliding interface with no real 'squishing' load. The applications are totally different and the thinner base 'leaks' by the rings at a higher rate.
In the early days of multiweight, I worked in a service station and later was involved in a lot of engine repair.
All my customers experienced higher oil consumption with multiweights. Usually they would burn no oil in the first 1000 mile, then in the next 1000 burn a quart. If it wasn't changed at that point, consumption would increase so that from 2500 to 3000 it would burn full quart.
That's mostly Phillips Trop Arctic, as they were one of the early makers and local to me. So lots of it sold here.
That's my story and I'm stickin' to it.