The issue is cold start and actual operating temp oil viscosity.
The oil will act like the winter grade number on start up and when at operating temp it will act like a 30. A 30 will be like a 30 on cold start and get thinner with temp.
The reason for single grade oils is that before WWII is that Viscosity Index Improvers were pretty rare, and what was available was sketchy at best. After the war, the VI Improvers because better and now are damn near fool proof. I say damn near because I've seen guys use oils with alcohol that weren't alcohol compatible and the results were bad at best.
