I think you're on the right track with heat, but I'm not sure if oil would be the culprit, the plugs were dry and had no fouling. Maybe I'm biased because I
really don't want to believe that my engine has any issues with rings, pistons, cylinders, etc.
I'm assuming the shell is made of steel, and I found a diagram of different colours that can be achieved through different temperatures when heat-treating steel.
