Ditto to what Dave said on overloading these engines, you can really roast a motor in a hurry with a lean run/overheating during flight.

What I found for propping is that it is really dependant on your altitude. I used to live at about 1000' asl now I live at 3500' asl. I spent a long time learning that the props that worked for me at low altitudes were way to much for me at higher altitudes. Although they seemed to run fine on the ground once in the air they were obviously loading the engine down too much. Now that I have it figured out I have no problems with my motors. I now dont get too worried about the rpm the engine is running but how it sounds in the air during a flight. If the engine sounds like it is working hard then it is and smaller prop will give you better performance and longer engine life.

So just be carefull about using props that others recommend as your location may not be that suitable for that particular prop, and you may be loading your engine down too much.

On a side note I have seen and experienced valves (exhaust specifically) that would be covered with buildup enough to affect the seal, but this should take far longer than 80 flights to occur, more like 400.