Okay, just to answer the original question, from someone who actually runs 2 OS max engines as we speak. One a .15, the other a .25, both lapped pistons that I bought for very cheap in the mid to early nineties. I've run both of them a lot over the past 15 or so years and have used almost exclusively Ritches brew and according to the label I run 50/50 castor synthetic blend with 22% oil I believe, I'd have to run out to the shed to see for sure, but I think it's 22%. And they both run fine. I've only ever had 1 gallon of pure castor fuel (fox fuel) and it was winter time and it was like cleaning syrup off of the airplane. And one gallon of pure synthetic (red baron brand fuel) and on that one I toasted the rear bearing in my Saito .50 and never bought another gallon of pure synthetic. But the Saito was an old engine, and I think the real problem was oil content, my flying buddy and myself put some in a measuring cup and let it evaporate and when we figured what was left was oil it was only about 15 to 16%. As far as plain bushing engines in general, my most run engine of all is my OS 40fp, I've used that engine almost constantly since about '92 on the 50/50 fuel and have never had any trouble, I like plain bushing engines because there is no bearing to rust and go bad. And last, I have a YS .45 that I have owned nearly as long, and haven't run nearly as much, mostly because I've had to take the diaphram off and clean gunk, and also sticky check valave, both probably due to castor in the fuel. That's my guess anyway, I'll clean it up put it on something after about 50 runs it'll become hard to keep running, then I take it off, put something else on the plane until I feel like going through the YS again.

Austin