High nitro engines run cooler when operated on the rich side of peak power. That is because pure nitro has more and more power the richer it is run. In fact it cools the engine very well, so well that the nitro dragsters need no cooling, the modern engines don't even have a cooling jacket. However on a lean run the nitro may approch its stoichiometric mixture which means the flame temperature will be higher than that of the methanol. The oil does little for cooling, rather, more than remove heat it prevents it by reducing friction.