I do not see how much oil could leak back to the engine when the fittings are installed on the bottom of the cooler. The oil would have to go through the oil pump gears, and the engine bearings, etc., have little clearance to allow oil to pass through.

Hi. Remember that a two-pass or worse, a three-pass oil cooler (or water radiator) does not necessarily cool better. When you pass the fluid through three times, you effectively pass it through three small coolers. This causes the oil to remain in the cooler three times longer. Remember that...