Hard to tell you for sure since we don't know what motor or car for that matter. I'll guess it's a 103 or 104 motor since they are the ones that like to leak oil into the coolant. Prime mileage for a head gasket. Check the right rear of the head at the block to see if it leaking oil there. Oil cooler is another possibility if this one has one. Don't put this off,it will only ruin the hoses and be harder to clean the cooling system after repair. I use Castrol Super Clean 50/50 with water, warm the engine up and flush it a couple of times. CSC doesn't foam as bad as dish soap and is easier to flush out.

fasthair