2 points here need refining
Oil IS coolant for the engine. If the film is maintained so well, why does an engine get hotter when you lean it out? (assuming you maintain the same RPM)

This is also true of gas engines that do not use premix.
The fact that oil is a coolant in model engines comes from the fact that it asorbs heat from the combustion chamber
and is expelled out the exhaust.

The large oil reserve in automotive engines does play a part in cooling.
But it is because it picks up heat from bearings while it is being pumped to those critical areas.
Of course it does pick up heat from other areas too.
But it is picked up ran through an oil cooler and then back to the bearings.

The larger resevoir , to a point, the cooler the oil will be.