$$$A large portion of the driving public doesn't care about oil and will not pay the premium price. It doesn't show the leaks, it just creeps past gaskets easier.
It will give you a little better gas mileage when you use the lighter weights. With the increased protection you can get away with thinner oil and the engine will have less to work against. They don't boast as much about it like they did in the 70s because the conventional oil has improved so much (the additives) that they now are using lighter weights. Years ago most used 10w-40 and Mobil 1 used 5w-30. Run your hand through a tray of oil (for a jolly) and see which viscosity is easier to go through.
