A catalyzed urethane will best stand up to gas & oil. I always use an etching primer after cleaning & degreasing. I'll also scuff the tin parts for good mechanical adhesion. With little care - just keeping them clean, basically - those motors look great decades later.
For the race motors, I paint 'em a light industrial grey; any leakage is quickly obvious, and it's also easier to see things when underhood. That's probably not for everybody.
I also like to see things looking like what they're made from, so some day, some motor's getting painted cast-iron grey.