I think there were two quantum leaps as far as oil cleanliness.
First of all, the move to unleaded gas (1976 here IIRC) was incredible. The debate prior to this was whether you changed your spark plugs every 10,000 miles or twice a year. More importantly, the elimination of lead in the gasoline has resulted in much better public health outcomes, and is arguably responsible for the drop in crime rate, starting in the '90s.
Secondly, the move to fuel injection helped a lot. It's rare to smell raw gas out of the exhaust of a cold car now, and when I do it's almost always an old carbureted car.