grease like sludge in radiator HELP!!

This 4x4 toyota pickup has brown grease like sludge in the radiator. The heater quit working and this is when we noticed a problem. The sludge is the consistency of grease. My thought would be a bad headgasket, but the oil is not milky. Could oil in the radiator even get as thick as grease? Do coolant and oil mix? Any thoughts are appreciated. Thanks.