Theres not much difference between 20-50 and 15-50 when HOT, they both behave the same.
I found about a 5 lb. difference in
oil pressure between 15-50 and 10-30 Gibbs Hot Rod
oil HOT. I also noticed a difference in temperature between the two. In my SBF the 30W ran 5 degrees cooler than the 50W. I'm staying with the 10-30.