Mercedes use several types of transmissions some have a synthetic fluid that they claimed was sealed for life but "changing the fluid" added longitivity to the transmission but use Mercedes approved oil. Some Mercedes have a fill tube that is at the rear passenger side of the engine compartment but is capped with a tamper proof plug and require a dipstick to check the fluid level. Other transmissions have an overfill plug at the bottom of the transmission, the fluid is checked at a certain temperature and with the engine running fluid should just drip out. Safest bring your car to an independent repair shop that specializes in Mercedes and knows the proper procedure and fluid to use.
you dont -- is you shifter sticky difficult at times to put into gear. Just my luck I made this $1200+ repair last month @ MB of LB -- parts required were the Shifter, Pin, and Lock. Removel and replacement are made inside the car. It did not look hard for a DIY, I was going to do it myself; however, I was not sure the shifter replacement was the answer. And I would not have not been prepared/aware of the lock and the pin replace. Hope you can diy and save - good luck