I used APC 16x6 while someothers use 15x8 prop. I put 3 17oz tanks of 7% nitro before flying. However, the manual states that the engine is pre-tuned for 10% nitro. Today I am runing 15% nitro with 15% synthetic, but still havn't even had a gallon thru it.
I realized that a tachometer is very useful tool to verify that the engine are running good as the recomendations is low 9000 RPM. Having that said today I reach 8600 RPM with 15% nitro and obviously is hoping for some more in due time. Read the manual before doing anything. My engine started the very first time on its very first compression and stayed running.
I love this engine and the sound is awesome. Eventhogh this engine is a bit on the heavy side it ticks like a swiss watch with good power and has the benefit of the built in fuel pump. I took advantage of the pump and placed the tank on the center of gravity location. I am certain that you will, as most on the OS 120 III owners, love this engine. Enjoy.

Regards,

C.