When upping the nitro content you needle valve(s) will have to be opened. Nitro basically carries it's own O2 with it and allows more Methenol to be introduced and burned = more power! Run times per tank will also decrease. Along with more methenol will come more oil so your smoke will also increase! I got some 14% oil 30% nitro from Powermaster to try out last year and it ran very well...it was nice to have the power of 30% and still be able to see the heli!
With the difference between castor and synthetic oils; castor is a thick oil that needs a large orfice to get to the motor...a thinner oil (synthetic) will flow thru a smaller orfice = less needle valve.
Hope this helps you. Chris