I always use the 18% oil by <font size="3">volume</font> fuel that is a blend of castor and Klotz that is recommended in the OS instructions. Only after it says this does it allow that good quality all synthetic can be used. It also doesn't say 18% by <font size="3">weight</font>, which is said to be done by some fuel manufacturers. I use Sig (actually 20% oil) and Omega (only after break-in, works fine in high temps).
That being said, I have a 46AX (11x6 APC) and a 55AX (12x5 APC) in my two Kaos's that run like tops. 4 Gallons through the 46 and 2 through the 55 and they're great, one flip starters.
I did read a thread about the pressure taps being plugged in some AX mufflers, that would make them go lean and quit.

A buddy has a Reactor with an OS 46 AX and an APC 12 1/4x3 3/4 prop that seems like magic in it's ability to 3D. Might be him, though. He chose the engine because of it's power to weight. That prop on a 55AX would seem killer.

Hope you get your's figured out.

Chris...