If you want good service from the transmission, the pan needs to be dropped and the filter changed. We're not talking about a complicated mechanical process here. Six bolts for the pan, and three screws for the filter. While the pan is off, a good mechanic will thoroughly clean out any accumulated residue from the bottom of the pan. Most often that will be very fine metal shavings.
For the do-it-yourselfer, it is important to note that the transmission pan gasket that Mercedes uses is a soft rubber gasket - easily split if you tighten down the pan bolts too much. So, use a torque wrench and the right torque values.