Do fuels with no castor just synth. oil burn cleaner?

No, not really, but the castor oil is what turns brown on your engine from the heat. Not sure I'd recommend fuel without castor, but I know people do use it.
Dennis-

Fuels without castor NORMALLY do not leave the brown, baked on residue seen on the engine. IMHO, the down side to an ALL synthetic oil fuel is the lack of protection for the engine in case of a lean run. The castor leaves a residue inside the engine that helps protect the engine from rust and lean runs.

or clogs up a 4 stroke if that is what you are running... (oh oh, another fuel war!) Anyway, ignore that last comment. [8D] There isn't much you can do about the fuel, what you can do is use an exhaust deflector. Some swear by them, I'm usually swearing at them. I've never had good luck with them. If it actually stays on for more then a flight, the reduction in power is noticable, but again, your mileage will vary...

It is quite easy to keep an exhaust deflector on. Use your dremel (or a small triangle file) and cut 2 SHALLOW grooves all the way around the exhaust tip of the muffler. When you put the deflector on it should not come off after that. I have several planes with exhaust deflectors on them. Since I put the grooves in the exhaust tips, I haven't a single one come off.

I use deflectors and after some trial and error I found the best and most assuring way to keep them on is to attach them then drill a 1/16th inch hole right through the deflector and the muffler tip. Then run a small piece of wire through the hole and loop it . Works great and havent lost one yet....especially at almost $3 apiece.

It just seems odd that we use castor oil. We dont really use it in motor oil anymore( there are exceptions) because it does not perform as well as synth or natural petro based oil.

Burn cleaner, NO
Burn without a residue, No
Has less varnish and is less sticky, Yes