The modeen is a small factory oil cooler bolted to the block .
Its not really a high performance oil cooler just a water jacket round the outlet oil pipe from the block and is designed to keep the oil temperature sensible .
If the oil pipe cracks/splits inside the modeen when the oil gets hot the crack/split gets a little bigger. Because the oil is at pump pressure it is pushed out of the split into the jacket and because water isnt under the same pressure it doesnt get back into the oil system.Obviously with a high power oil pump it will be worse
As the split is usually small the above only happens when the oil gets hot and this can make it hard to diagnose.
As the oil is lighter than water it goes round the system with the water but tends to get stuck in or at the top of the rad
I have never ever seen the oil/water cooler split in working on these cars for nearly 20 years.
We repair the head on site , can turn then around pretty quickly. But the head needs strip right down to a bare head.
We had some many people think that the oil in the water is a head gasket problem , oil/water cooler problem but it is always the caracked head problem.
The blocks only crack externally , which is normally above the ridge by the starter motor area. a lot of 1996 onwards blocks go porous around the oil feed connection on the turbo side thou.