I don't know what engine you are referring to, but most four stroke manufacturers recommend synthetic oil to prevent gumming/sticking valves. That said, it probably wouldn't hurt to use a blend for your break-in only. The medicinal spec castor is usually not recommended. Go with a good brand of commerical like Klotz. I have a YS .91 AC which I run YS blend 20/20 which is synthetic. You will find that most four-strokes require more nitro (15-20%) to run properly due to the time between compression strokes. You also need to use an O.S. type "F" or YS four stroke plug or I assure the engine will NOT run at less than 1/2 throttle.

Hope this helps!

G~