Oil viscosity - why the 20W-50 BS?

The car I bought from a friend's widow just had a purple 508 cam installed. The guy never heard it run before he died but he did leave a note saying to use Valvoline 20-50 ZR-1 with zinc. The local speed shop is a Mopar house and they carry and recommend Brad Penn 20-50 green oil which I ran in the stroker a while. I was noticing that the oil pressure is getting slow to rise and the engine is very cold blooded on first start after sitting for a week or so. I put in 10W-30 Brad Penn this oil change and the cold blooded nature went away mostly and I still have 60-70 pounds at high idle after the cold start.

Why the heck are people telling us to run that thick oil? I believe it is hurting the engine especially when cold. In fact I have started the practice of turning the engine over without spark until the oil pressure comes up. I use a drain back blocking valve Purolater oil filter.