I would strongly suggest removal of the cyl. head to check the flatness of the head, because the right rear cyl. area is a common area for oil and coolant leak on theat engine. At that time, it may not be a bad idea to check the back sides of the valves for any excessive carbon/oil buildup. Also, the replacement of the water pump would be another idea at that time whilr the head is off.
Paul R. 21 year MB tech.
MB master and ASB tech