I'm using a castor/synthetic mix. It's harder to clean off the plane, but careful attention to where the exhaust is positioned will help a lot in this regard.

Also, I've been using 16% oil, but I recently mixed a gallon of 30% nitro, 22% oil heli fuel with a gallon of Byron 15%. That gave me about 20% oil. I've noticed the following:

* Nice trail of smoke from the engine.
* Engine is much quieter
* Run times are slightly shorter, but not much

I'm thinking about sticking with higher oil content even though I never had any problems with less oil. I think the engine is happier and seeing that smoke trail makes me happy.