20/50 won't make it fail, but it is thick, and the engine will slow down and work to churn through it. He has a 318 not an overpowered hotrod. have you ever tried to fun in a swinning pool?
I mistakenly put 10/40 in a small 4 cyl econobox car. It would work to go 60-65 on flat ground. I looked it up and 5/30 was the oil for it, and it drove very different with thinner oil. It does make a difference.