I recently discovered a thick coffee-colored.

I recently discovered a thick coffee-colored substance in my reservoir that appears to be oil. It's thick and sticky when cold, running & thin when warm. There wasn't any water on the dip stick, no check engine light, no loss of power, no smoke/water blowing out the tailpipe. I was able to drive the vehicle for almost 2 weeks before the substance got too thick to allow the water to flow through the motor. Some people say its the head gasket, some the intake gasket, one person said it was the residue from the motor and that it'll take about 2 months for it to flush completely out of the system. One suggestion had to do with the heater core. We've flushed the system with flush cleaner twice and ran water through the system numerous times in attempts to get the gunk out of the cooling system.
What is this stuff? How do I resolve this problem in the least expensive way?