Oil in coolant
1989 BMW 3251 E-30 4-door. Running fine but oil in coolant. Beige colored sludge appearing in coolant reservoir. However, no coolant in oil. Suspected head gasket. Pulled head. Gasket showed no sign of leakage. Had head pressure tested. Tested fine. No cracks and not warped. Prior to this, I had begun to have over-heating problems. I had to keep adding coolant, too, every 50 or 100 miles. I got a new cap for the expansion tank. No change. I couldn’t find any leaks. I asked a mechanic and he said something about possible exhaust gases doing something or other which I didn’t understand. Another mechanic said that if it had become severely overheated, the head might have temporarily warped enough to allow some oil into the water jacket. I’m at a loss. Anyone have any ideas?