For the underbelly of the beast, original Go-Jo hand cleaner (not the stuff with pumice), works amazingly well to remove oil, grime, and exhaust stains. Wipe it on with a rag or hand, then hose or wipe off with damp rag.

I tested Go-Jo by sticking a piece of aluminum half way in and waiting a couple of weeks to make sure there were no untoward effects.