Definately use both, they have very different properties both of which are necessary for good performance. Synthetic oil burns almost completely away in the combustion process where caster doesn't burn leaving it to cool the engine. there are several forums discussing fuel mixing on rcgroups.com and rcuniverse.com. Plus here is a couple of links that have good info. Be careful - not much to gain and a lot to loose. Good luck!


http://rcboat.com/fuelmixing.htm

http://www.lcrc.org/stu11.htm