I have an M1009 CUCV with 6.2 diesel. When draining the cooling system I noticed what looks to be oil in the coolant. There is no white smoke or liquid coming from the tail pipe and no coolant in the oil. Engine does not run hot so I don't think it is a head gasket problem. The oil cooler lines run outside of the radiator except where they enter the radiator for about three inches and then out so I don't think it is that. The thermostat had also turned dark. Any ideas?