Oil viscosity - why the 20W-50 BS?

my friend has a Hemi Dart and runs the heavier oil to keep the engine cooler, well mostly the bearings due to large clearances, using light oils on cars with large bearing clearances tends to "blue" the main bearings, aka f***ing them up...

I guess it's logical but we run 10-30 in everything we own and haven't had problems yet