I am always amazed at the damn fool crap you can read on forums. Some of the people responding are ten and have never owned an engine. Most of the people responding have never owned a YS engine. You get the answers from 40 years ago - C.F. Lee. YS engines are full of silicone rubber seals, O-rings, diaphragms. Performance Specialties (Dave Shadel) AF oil is specific to YS engines. Mobile 1 and Mobile 1 pure synthetic ATF will not harm silicone but may not be a good after run oil. Castor ? Really ? Richard Verano loves you guys since half of his business is repairing engines that were pumped full of after run crap.