I've been using PowerMaster 10%nitro with 18% Castor oil.

Regarding the muffler, I have had lots of problems with it coming loose. There are two parts to the muffler, and the back part from day one would rotate when running the engine. I tried to tightening the screw that runs through the muffler and add lock-tight, but it didn't fix the problem. The threads became worn. I then bought a new muffler (same one) and that worked for a while, but it too started to become loose and the back part would rotate. I then put a threaded rod through the whole muffler and used two nuts at both ends to tighten it down. This seems to have fixed this problem.

The fact that the muffler keeps vibrating loose seems to indicate excessive vibrations from the engine.