Originally Posted By: kschachn
So if the heat of combustion is being more absorbed by the oil, where is it being rejected to the environment?
Through the oil sump and oil cooler.
Besides the heat capacity of an oil there is another property described as fluid thermal conductivity. For a given flow regime (level of turbulence, ie. Reynold's number) some fluids convect heat from the core of the flow to the boundary more or less effectively than others, and can thus accomplish the same amount of heat transfer with a larger or smaller heat exchanger, or an increased or decreased flow rate. I would posit that heat exchange properties for bases used in motor oils, esters > PAO > mineral oil. I do not have numeric values that quantify the actual rates, but I believe some difference exists.
In the water-cooled motorcycle instance, the exterior of the crank case would be the same temperature when transfering the same amount of heat per unit of time to the environment, comparing an ester to a PAO to a mineral oil. However, the ester motor oil in the sump would have a more even temperature distribution from the core of the fluid to the boundary where it contacts the crankcase case because it is a more thermally conductive fluid. A mineral oil would have more temperature delta between the core of the oil and the surrounding crank case because it is a less thermally conductive fluid. The same theory applies to locations where oil is absorbing heat energy in an internal combustion engine.
I believe that is the main mechanism that accounts for how the temperature difference phenomenon occurs, but I would like to listen to your argument for why there can be no difference.