Fours produce more useable thrust from tourqe, two strokes produce torque at a higher rpm when you enter the power band curve, but unfortunately, the prop slip will eat thrust when that happens. The most efficent prop is going to be a slower turning high pitch prop over a high speed low pitch prop. As far as the four stroke debate, they all operate in the same principal, the only one's that deviates from this traditional setup is RCV and HP, Hp used a rotary valve for intake and exhaust, called the VT series, and RCV does the same thing, but spinning the cylinder to acomplish it. RCV made a early design that ran off the cylinder or as we americans call it the "camshaft" that runs the valve train, but since the port for intake was the cylinder rotating, there are no camshafts or valve's, just a spinning port. The latest design is the same, but they use the crank as a output like a conventional engine, and I have one in my skybolt, it runs like a top and is very reliable due to the plug being shielded from fuel and oil till it's ready to fire via the port timing of the spinning cylinder. The low profile design is a perk but the weight is a negative, alot heavier than an conventional 4 stroke engine. Adding excess oil to a fourstroke prolongs life, but kills plugs and causes loading-up issues, running too low a nitro will not produce enough heat to keep engine at running temps, nitro adds heat, keeps elements red/white hot and since there is a longer duration between combustion cycles, the more heat the better it will stay lit for the next cycle. Generally low nitro will cause idle issues and loss of rpm, extremely high nitro will cause predetonation and excess heat buildup that the engine can't and wasn't designed to get rid of. Most 4's will run fine on 10% and much better on 15% nitro, only power junkies really see the difference using 20 to 30 % nitro. For most field flying, 10-15% will suffice, another aspect that I'm not looking to start a fire on is oil, I've been running everything with a little castor oil in the fuel or at least 50% castor, I look at it this way, if I have to wipe it off the plane, then it's lubercating the engine, if it vanishes, it was burned off in combustion and provided little or no help to the engine.