brown sludge in radiator

i have been finding brown sludge in radiator and overflow bottle, truck is 94 burban with 350, 180,000 and the oil cooler in the radiator. no coolant in oil pan. i've asked my buddys and have gotten the oh you blew a head gasket. but i dont think so. i think its the radiator, so how about it guys anyone got an ideas. dont want to pull the heads off to find out it was the radiator. going to remove the oil cooler from engine and flush coolant and try that to see if it is the radiator but anyone got any better ideas?