I don't remember in what manual, because my Grey Market '84 Euro 240D did not have an owners manual. But I found, I believe in a Haynes manual, that it took engine oil. I put in 10W30 Mobil One, and have changed it a couple of times. The car now has 533,000 miles and the four speed manual has never been apart. It has only been out of the car for three clutch jobs. It still works and shifts great and produces no noise.
Modern ATF has evolved to a point where the additives are not good for seals other than those found in automatic transmissions. ATF is definitely not good for seals in your power steering system. I would want to determine that ATF is DEFINITELY what MB specifies before I put it in your manual transmission. The lubrication and weight would probably be okay, but if it ruins a seal and leaks out, it doesn't do your transmission much good.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in