You guys and your WD40....[&:] Using after run oil isn't a bad idea, but only for long term storage. If you run the engine dry and empty the fuel tank, no ARO is needed, generally. Using fuel with castor oil in it will protect your engine superbly! I put the piston at BDC so the crankcase can breathe and you're good to go. Never any rust in my engines.

Petroleum based anything in a model engine is hokum. I tried starting an engine that I used petroleum based ATF as ARO and it was the biggest PITA to get started. Never again will I use petroleum based anything for ARO.

I've been modelling for 16 years and have yet to get any carbon or "deposits" in an engine.

Oh yeah, Did I mention WD40 sucks?

If someone gets carbon buildup, run the mixture rich. The extra raw fuel will loosen some of it up. Though it rarely affects engine performance unless there is a lot of carbon.