FlyerinOKC, I have found that running 15% in Saitos is ok unless you are try to screw the last amount of performance out of the engine. Then more oil is needed and it is why I suspect that your Saito distributors in the USA say you should use castor while elsewhere they don't. It is the same with my YS 53 and 63's. The castor gives you a bit better protection but now that Saito actually market / recommend a specific oil I guess that will change. I run a minimum of 20% synthetic and and have yet to have engine stress problems due to lack of lubrication in either my competition or sports engines. That a/c you have on your id page is that a Bellanca or an Airspeed? I have a long standing desire to build both.