Seems like I replaced mine with regular gear oil recommended for trannies, and I did consult with Mercedes when I changed it in my 1976 240D with 4-spd. I never heard of using ATF, as it's soooo detergent, it might eat seals a little sooner.

Where you intend to drive the car is what will determine which of the two viscosities you want to choose from. You can also use syntec. But I used the lower viscosity (rather than 140), as I didn't want it to become the thickness of vaseline during the Montana winters . . . LOL. When I drained it, it was also clear that it was not ATF, but regular gear oil (80 viscosity???). What is in it now? If Synthetic, replace with synthetic, if universal gear oil, use universal, but never, never, mix them (will over heat before you get to the freeway onramp!!!). Check to see what is in there now, I would bet 80-90 viscosity oil, maybe synthetic. It'll be amber/brown. ATF back then, as is also now, was pink in color.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002