I just completed a head-gasket R&R on my 1990 300SEL. I had a similar situation. I had both oil and coolant seeping from the head gasket on the passenger side of the car. Mine was more toward the rear of the block. No oil and coolant mixed in the engine, and I never had an overheating problem. If it is a head gasket leak you should be able to look up, from under the car, on that side of the engine at where the block meets the head. In my case there was a thin trail of oil and coolant and white effervescence clearly leaving the leaking area of the headgasket.