Oil cooler is failing. Oil pressure is much higher that coolant pressure. You need to get that fixed ASAP. I would say there is a bit of coolant in the oil, just not enough to see. Because once you shut the truck off and the oil pressure relieves, the coolant pressure would be higher and could cause mixing. Unfortunately, sounds like time for an oil cooler replacement. I would have the EGR Cooler checked also, or, if you have no emission testing, have it deleted......