Probably oil cooler o-rings have gone bad and hardened up. Need to drain coolant... Remove the cooler and replace the o-rings on both ends. I have never worked on a psd but its probably the same thing. May need to lift the motor for clearance. After all this is done just run some Dawn Dishwashing detergant in the water for a few miles. Mostly the heating up and cooling down of the coolant will force all the oil into the coolant overflow tank. Its easy to get it out of that. When the system is clean drain and flush several times and refill with distilled water and coolant.