We have no suggestion for a fuel/oil mix when you're running gasoline. The engines weren't designed with gasoline in mind, and we've never run them that way.
As far as using modern fuels with little or no castor oil, just make sure you're running the engine on the rich side. The castor is best when you have low oil content or are running hotter than usual. We'd most certainly add some castor to fuels that have none. If you have problems getting castor oil, you can most certainly use medicinal castor oil from the drugstore, or purchase Klotz BeNol through hobby outlets or racing shops:
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