I agree with Jim, oil takes up space in the combustion chamber and gives a slight increase in compression. Yes, oil can vaporize and burn but in order for that to happen, the temperatures have to be over 500 degrees F. In glow engines most of the oil just passes through the engine and comes out of the exhaust, that's why we need the Formula 409 and the paper towels after we are through flying. In extreme cases of flooded engines, connecting rods have been bent because of hydrolock from all of the incompressible liquid in the combustion chamber.
Some worn out old engines may run better with high oil fuel because the oil is helping seal the compression.