Different mechanisms for oil cooling in glow and gas motors. In a glow motor, the theory is that the 18% oil soaks up some of the heat from the motor and carries it out the exhaust port. That's besides the effect of oil reducing the friction. If friction goes up, power goes down and heat goes up.

In a gas motor, there is not much oil (1-4%), so the oil is not going to transfer much heat out the exhaust. However, that little bit of oil has to provide lubrication to reduce friction for the rolling element bearings as well as between the ring and cylinder. The oil film also helps the gas seal for the ring. If you lose the gas seal, then hot combustion gas goes past the ring and overheats the piston. The oil also helps transfer heat from the piston to the ring and then to the cylinder.