My '92 M103 has had a headgasket leak for 130,000 miles. It used to be a combination of coolant and oil. Then I switched back to dino oil from Mobil 1 and the oil portion mostly stopped. I can crawl under the car with it idling and see the leak. It is very minute, but kind of spits from on point about
2/3 rds of the way back from the front of the block on the passenger side.
Requires topping off the coolant about every 400-500 miles. I have no oil in the coolant reservior.

Crawl under there with the engine running. Shine a strong flash light around the engine. Maybe get a telescoping mirror. I can't believe that you won't find it. Look at the bell housing of the transmission. Do you have any oil dripping on it.

Steve