The only problem with using castor is that it builds up on the exhaust valve. Synthetics do it too, but not as fast. Castor becomes a thick goo after leaving the engine set for the off-season, so prepare to be gooey come Springtime.

You really ought to use the recommended fuel, 20 synthetic, 20 nitro. Lots of mfg's make it under the YS 20/20 name.