Jan,
There are actually a lot of fellows in SAM that run 15%, 100% synth oil, in the cox engines - me being one of them. As long as you are running a larger prop (7 or 8" diam) that keeps the rpm's down, they do run really well (I have many hours on certain engines with absolutely no adverse effects). However, if you are running a smallish prop for high rpm, or tend to set your engines on the lean side, I suggest you go with a bit of castor, if nothing more than for the ball/socket joint. Just my opinion...

AJC