I use 15% Power Master with 18% blended oil in all my four strokes, even the YS engines but I don't think the nitro percentage is a problem. I use the rpm drop when I remove the glow driver to help me set the low ends. The rpm drops when the low end is a little fat. I don't see it as a warning that the plug is weak or ready to go bad. Plugs just happen to burn out over time, sometimes fast. If the engine is new they can short out from the metallic crud as the engine is breaking in. Pop in a new plug and see how many flights you get out of it, I get a couple years worth. I don't recall ever having a plug burn out in flight but it has probably happened to me? Most the time the plug goes when I'm starting the engine and I don't know it until the driver is removed and the engine quits. There is a glow driver that can show you the plugs condition with LED lights, a lot of pattern pilots use them but they aren't cheap.