I followed my manual and used Mobil 1 10W30 last year. PO had some kind of transmission fluid (red) in it. So far so good - It reduced the notchy shift from 1-2 and the shifting feels nice and substantial now. Plus, it's really inexpen$ive

Next time, I may try the redline MTF. Others on here have had nice things to say about it.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT