Oil viscosity - why the 20W-50 BS?

Someone said it in one of the posts - it all depends on the motor and build of the motor. Older motors are not made the the tolerances of today's engines so you can't compare the two. A stock or close to stock build of an older engine probably does not need a heavy weight oil and does not benefit from it. The caviots to this center around temperatures and use. If you drive around in MN in 30 below zero weather, you definitely don't want to push 50W through the engine. If you drive around in Houston in 100 degree weather 50W may be best. If you run the engine hard in hot weather heavy is better. Then you get into high performance engines, loose tolerances, high compression, high rpm use - heavier weights are better.
One other thought - if you run a motor that does not have catalytic converters behind it - run a high zinc oil. I don't care if it's a mild build, hi perf, roller or hydraulic, use the zinc. If can't hurt and can only help... My $0.02!!