Just use a normal commercial degummed castor like Castrol M (probably the best there is) although I think some of the American brands are OK too. Bakers AA may be similar to Castrol but Klotz (according to their web site) has a few other things mixed with it. None of the synthetics will help give better engine life than castor.

As for avoiding carbon and wear by using a straight synthetic, you've got far more chance of damamging the engine with just synthetic. All it needs is for the muffler pressure line to fall off and you've got an instant mega-lean run. Synthetics protect in normal operation but fail instantly when something unusual happens. A 50/50 mix is fine though.