Missileman ... even synthetic will cause the valves to stick a bit but less than castor. Saito and YS do not recommend castor. Castor also leaves a residue, its lacquers the parts. I do not use anymore castor as its messy but if you fly often the engine should not really be affected. However, the gumming up and lacquering will build up eventually. YS do not recommend castor as if may affect the pump system.