Get a can of Gunk, and get your local Dyno Rod drain cleaner man to blast it off with his high pressure water jet - you need your engine out for this, though, as the pressure they run at easily strips paint straight off. My mate did mine for me yesterday, and it got rid of 15 years of oil and muck in no time at all. And my engine was DITCHED! To give you an idea of how powerful it is - my mate said it can easily cut through your hand if you aren't careful!!!!