My '82 240D shows no tranny problems at 260,000 miles.
I have a friend with an '85 300D with somewhat over 500,000 miles. He has had the car for the last 400,000 miles or so. I believe the tranny is original, AS IS THE FLUID. Two or three years ago it began to make a whine sound that turns off and on. No shift problems. He was told it was " a gate valve". Neither of us knows what that is, but the noise has't changed in over 200,000 miles.