Im pretty sure its the HG but maybe just by chance (cross your fingers) it could be the oil cooler. The truck runs fine no problems no white smoke out the exhaust i just noticed i was low on coolant so i put about a gallon in. the next day i notice my resivoir is black and the radiator has oil in it. but like i said the truck runs perfectly fine. Anyway n e one know of a way to test it to narrow it down thanks alot.