Oil viscosity - why the 20W-50 BS?

I had always been told that the only reason to run heavier weight oil is to permit you to open up the clearances in the motor. A couple of machinists I've talked to concur, that if you take an extra tenth when you turn a crank, you reduce friction in the motor by better floating the crank and rods in oil. The upside, better rpm capability, less friction, and less wear under heavy duty use. The downside, you have to run heavier oil to keep the pressure up.

It's the same reason some drag racers around here pull worn out junkyard short blocks, 50 weight oil H/C/I and a 200 shot, and run 'em 'till they blow.