In reply to BimmerMaven :
In terms of how much heat they remove, any oil-to-water cooler that's sharing a coolant with the engine is going to remove less heat than an oil-to-air cooler, they also tend to have higher pressure drop than oil-to-air. If the exchanger is big enough and has a dedicated liquid-to-air system it could match or exceed the cooling capacity of an oil-to-air system. Any oil-to-water cooler can potentially fail in a way that causes coolant and oil to mix and it's the most common failure mode.
