Colder oil has much more friction than oil at the proper temperature. The question is "what is proper temp". Max oil temp is usually stated in an operators manual, but normal operating temp may or may not be listed. I suspect that raw water
cooled engines could gain some milage if there oil ran at a higher temp, but fresh water
cooled engines almost always have an oil cooler in the system that keeps the oil from getting too hot. What most people dont realize is that when at start up the oil cooler actually heats the oil up to operating temp quicker than if there was not an oil cooler. The oil cooler or heat exchanger
uses the fresh water
part of the system to keep the oil temp stable. When you are first running an engine
the coolent water gets up to temp before the oil does, so the heat exchanger
transfers that heat to the oil to bring it up to normal. This cuts down on friction and wear on the engine
, which makes your engine run better and last longer. I am sure someone can give a clearer explanation than this._____Grant.