I have engines that are over 4 years old and have little of the brown varnish or anything else, and I don't ever clean them (unless crashed, and then it's just Isopropal and carb cleaner spray). One of the biggest reasons for the varnish to form is running too lean, the engine is getting hot and cooks the castor that is on it. So if you see it starting to form, you may well be running too lean.