Brian A wrote:
I've always been unclear about the purpose of an oil cooler. Is it to aid in engine cooling? Or is it to just to keep the oil below its maximum operating temperature?
I think the answer is yes
Stuff like the cam, crank and pistons are oil cooled. so oil certainly places a role in cooling the engine.
...but then oil flows though passages in the block so the hot oil will pass some heat to the block which in turn passes it to the radiator. The temp of the oil will increase until the transfer of heat to the block equals the transfer of heat into the oil from the crank mostly but all the stuff the oil feeds.
This is kind of uncontrolled so oil temp will vary quite a bit depending on engine load/rpm/etc. oil viscosity changes with temp so the ooil's abilty to protect the bearings is also uncontrolled. The basic answer is pick an oil thick enough to work at worst case use and live with the fact that its costing extra hp to pump and doesn't lube as well most of the time.
A dedicated oil cooler with a thermostat controls the oil temp so you can use a thinner oil that is better under normal conditions and know it will survive at worst case conditions...because you're actually controlling the oils temp so its at optimal temp once the engine is up to temp.
Just adding an oil cooler in the air (or water) stream lowers the oil temp. Still uncontrolled but lower than it would otherwise be so you can get away with thinner oil. Car companies that want like a 0w20 for cold startup protection might opt to throw on a cooler or if you're already running 20w50 and seeing temps it's probably not happy at a cooler is needed.
The setup I used to have with the oil cooler on the hot water side is also an oil heater to help get the oil up to temp sooner. The "cooler" will be at at least 170C and become a heater during warmup, then become a cooler. It needs to be larger setup this way I think since the sink temp is 170-190C not 20-30C air temp when cooling is needed but it can provided really stable oil temps which translate to stable bearing protection.