Recommended oil for 360 magnum

I have a 300hp crate 360 magnum that my father purchased brand new in 2002. This motor has around 20 thousand miles on it. I live in Idaho where in the winter temps can get as low as 30 below zero and winter is the longest season. Summer it can get as high 105. I have been running 5w-30 in it and I had some people at bob is the oil guy recomend running a 0w-40 oil.
My oil pressure with the 5w-30 is 20 psi hot in neutral 15 psi hot in gear and while cruising down the highway at 65 it's at 45-50 and it will go up to 60-65 psi when you poor the coals to it.
Should I stick with a 5w-30 or 0w-30 or would going to a 40 wieght oil be ok?