There is a large range of nitro content available for your engine. However one concentration (or percentage) will outperform the others the trick is finding it. The larger the engine typically less nitro will be required. A .40 will probably run best on 10% while a .074 to .15 will probably run best on 25% to 35% nitro. If you add more nitro to say a .40 size engine going from 10% to 20% you probably won't see a performance boost but no doubt the engine will run hotter probably too hot 380-400+ degrees F. The Best way to tell when you have reached optimum nitro content is by using a tachometer and some way of measuring the engine temperature. (Raytek makes non contact temp gauges that give instant real time digital display read-outs). When RPMs stop going up and temps are below 380F you probably have the optimal nitro content. There are some other things to look out for. If you are burning glow plugs you may need to switch to a cooler plug (you might want to read this about glow plugs) You may need a new carb needle setting when changing nitro content. You want to be aware of "engine unload" some (most) engines need to be set a little rich when the plane is parked on the ground, but as the plane accelerates out and enters normal flight the engine leans out. Adding more nitro to your fuel may give a RPM boost by leaning out the run but the plane may not fly that way. When you add more nitro chances are you may need to fatten up the main jet. Another test is to just fly the plane when you stop noticing a performance boost you are running to much nitro.
Oil: Caster vs. Synthetic
Each has its pros and cons. Caster oil on the plus side can resist up to about 150 degrees F more than the best synthetic oil before it breaks down. Caster is an excellent rust inhibitor. On the downside it can and probably will leave carbon build up on the piston especially if its low grade. It also makes more exhaust mess than synthetic. Synthetic on the plus side mixes a little better with the fuel than the caster oil does, it makes less exhaust mess, and will not leave deposits like carbon around the piston. On the downside it does not stop rust and nitro promotes rust to form. It also breaks down sooner then the caster oil. Your best bet is to find a fuel that has both caster and synthetic oil so your engine can benefit from both.