Motor oil weight.

Hi gang.
I have a 2002 5.9 in my Dart. Don't know much about the motor. Bought it from a customer after I took it out of a 71 Cuda. Seems to be around 350 hp.
What weight oil are you running in your HP Magnum motor? In my 60-70's motors I run 15-40 Rotella but have been running 5-30 in my stock Magnums trucks and have over 300k on them. I was thinking maybe 5-40 in the Dart.