I'm by no means a chemical engineer, nor even a knowledgeable mechanic, but at 150K I'm defining my 560SL as "high mileage". Valvoline's
PR for their MaxLife product talks to additives to condition seals and reduce consumption and deposits--all of which make sense to me.
As far as viscosity, I do have the owner's manual, and I stick with its recommendations for the temperatures involved, going toward the heavier of the options given.