The owners' manual says change the trans fluid at 80,000 miles. I had it done at the dealer last year and they only put in 4 quarts so I'm figuring that they just dropped the pan and cleaned the filter and refilled what was lost from the pan.
Sounds like it. When I flushed the transmission on my ML320 (same transmission I am pretty sure) it took 18 quarts before the the fluid was coming through clean.