The rear main seal is at the back of the engine. In many (most) cases, oil leaks elsewhere make their way back there, so before worrying about that, look for and fix leaks at the valve covers and front of the engine.
Rear main seal (RMS) leaks are fairly common in the M112/113 engines as they age. I just did one in my 2000 C280 (190K miles) and have to do one in my 1999 E430 (63K miles) and my son's C43 (85K miles). The Service Manager at the local dealership tells me it has become a regular item. The seal is rather different from what most of us are used to seeing. It is a weird material, has no garter spring and some grooves that may act as a labyrinth seal. It also requires a special installation tool (it must be set 1mm below flush with the end cover) and the end cover must be removed to get the seal out. I questioned that but after looking at the end cover I see why the procedure calls for removal.
The dealer quoted $1,300.
The tool costs the dealer about $450. It is in three parts and also works on the older M104 engine.
I also renewed the transmission front pump seal while I had the transmission out.
These engines are common of oil leaks after you hit over 80K miles on your car. Seep isn't as bad as a leak...so, it depends on how bad it really is. Usually, I note it down and not worry too much if it's just a seepage, but if it's puking out and leaking badly, then I recommend it. Valve covers are also common on this cars after 80K.
I'M A FACTORY-TRAINED MERCEDES-BENZ MASTER SYSTEMS TECHNICIAN, ASE (AUTOMOTIVE SERVICE OF EXCELLENCE) MASTER AUTOMOBILE TECHNICIAN, BOSCH MASTER TECHNICIAN, MECP (MOBILE ELECTRONICS CERTIFIED PROFESSIONALS) MASTER TECHNICIAN, AND A L1 ASE & BOSCH ADVANCED ENGINE PERFORMANCE SPECIALIST WITH A MASTER'S IN BUSINESS ADMINISTRATION.
Seems like after the tech mentioned about the seepage...I have noticed on two different occasions now that the car kinda kick into a neutral mode when I am doing around 10 to 15 mph as I am slowing down and off the gas pulling into a parking space.
There are no noise of any kind when this happens...just a quick feel like it is not in gear. No messages on dash and when I drive away and punch it..it still shifts like normal and still hauls ***.
I have had the transmission oil and filter replaced at around 70K and the car has around 90K now.