In answer to the guy's original question, about the crap left on the airplane, if your needle is set correctly so you are not running lean and vaporizing/burning off some of the synthetic oil in your fuel, the amount of goo left will be exactly in proportion to the fuel's oil content.
The real reason for the description of synthetic oil being "cleaner" is that
1.) when over heated
2.) used oil is allowed to sit on the aircraft
bean oil (castor oil) goes thru a chemical change that becomes a thick, viscous deposit that is sticky and hard to remove. Synthetic oils do not and are almost always easier to clean off,
In the years that I flew glow I always mixed my fuel with some castor oil and some (mostly) synthetic (Klotz KL100.) The castor was no problem to clean as long as you did so at the end of the flying day. A mist of windshield washer fluid helped a lot. Was only a problem if you let the castor residue sit on the airplane more than a few days or if you didn't run some after run oil through your motor before you retired it for the winter. The real yuk factor was when you used fuel with no synthetic. The all castor residue, especially from a high, all castor, oil content fuel like Fox superfuel (which I think was 25% oil) was like trying to get honey off your plane and God help you if you missed any after that last flight of the summer!
Dave Hickey