Although the castor that is in contact with the metal of an engine won't burn, the castor which is part of the swirling combustion gases will.

A small amount of that carbon (from the burnt castor) will likely be laid down on the head, piston crown and exhaust valve.

A good synthetic seems to help keep the engine clean by stopping relatively small percentages of castor from sticking to the engine's internals when it gets converted to carbon during combustion.

As Asmud says, 2-3% seems to be "just right" as far as castor percentages go. There's enough to give a worthwhile improvement in film-strength but not so much that carbon will build up at any appreciable rate on valves and other hot parts of the engine.