Castor oil's great benefit is that it's got a higher boiling/decomposition point than the synthetics do. It adds what we call "lean run protection". Our engines are, of course, lubed by the oil in the fuel. As we lean a mixture, besides the engine gaining power, it also begins to loose lubrication, and make more heat. All this becomes critical at some point, and the heat of the engine's components is enough to begin burning oil as well as fuel. Synthetic oils burn off about 50 degrees lower than castor does, and it burns pretty cleanly, leaving little behind in the engine. Metal to metal contact occurs, followed quickly by seizing. When castor oil approaches it's temperature limit, it doesn't burn cleanly. It forms a slick varnish coating on the hot parts. This provides some degree of protection to the metal, and while the engine stops, it is likely to sustain less damage in doing so.

This is also why castor leaves planes dirtier than synthetic does. Some of the synthetic oil is always burning, and burning oil's useless to your engine. Oil that comes out the pipe and slimes your plane lubed the engine first.

Synthetic oils are entirely adequate so long as the engine isn't leaned too far. That's why we tune an engine by first leaning to peak RPM's, and then riching it to fall about 2-400 rpm below peak. Keeps combustion temps in line, and some (enough) of the oil survives the process.

J