For really stubborn exhaust and oil stains, I've used "Soft Scrub" for years. Any supermarket carries it, it doesn't seem to hurt the finish, at least the Poly Tone I have. I'm always surprised how long it takes to wash an airplane as compared to a car, but once you figure there is a lot more surface area then a car (a buddy who paints cars did a plane for me once, he was impressed with the sheer surface area on even a light plane), it makes sense. Trucks stops sell lots of trick washing aids for the big rigs, I love my extendable handle brush. It's kind of fun to bitch about, like how much work it is mowing the runway, we are lucky bastards! I wash mine only before attending major fly-ins, its a lost cause the rest of the time.