I did the transmission service this morning and everything turned out great, so far. The R350 runs great after the fluid change. My car has 67,000 miles with no service yet on the tranny. The fluid was black and the pan had a good layer of swarf. I dropped the pan and replaced the drain tube, filter, gasket and pan bolts. I measured out the fluid and got about 4 quarts so that is what I replaced with my new trusty pump. I did not drain the torque converter and am really not sure if there is a drain. Pumping in the fluid could not have been easier. It is about 35F here in Tennessee and my infrared thermometer read 40C and I got a little tickel of fluid out of the drain tube when I did the overflow measure. Putting in the amount I got out, I figured I could not be more than 1/4 quart off from what was in there before the change.
Not a difficult job, but I did spend about $350. I do have enough fluid for the next service and the pump, so the next time I might spend 40 bucks. While I was at it I serviced the diffs and the transfer case, which on a Mercedes is simple. I plan on doing this again at about 100k.
Thanks again mbdoc for the tip on where to get a fluid pump they are pricey but necessary for the job.

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tgmiller77
1997 Land Rover Discovery 158,000
1995 E320 Wagon 132,000mi
1999 E320 Wagon 97,000mi