I agree and that's what I run, an oil cooler.

Funny though we have 3 big blocks on the go, 2 strokers 1/2 fill and a 440 NO fill, the 440 is running the hottest on the water side and we are trying to sort this right now.

One way I was looking at it is if you have fill the oil down the bottom is not getting as much cooling from water and the water is not getting as much heat from the oil.