Why use ATF?
Well, I'll tell ya. I really didn't know either until I was reading the 5 speed ZF repair manual one day. In this small book it clearly stated that only ATF can be used because gear lube contains zinc as an anti wear additive and will harden the gears making them chip from going brittle.
OK, it didn't say that exactly but it did mention how gear ube will harden the gears causing them to chip.
Most of the parts for these rare transmissions aren't available or are extremely exspensive
($1,500 for a set of gears) ZF only supplied about 1,500 five speeds to MB which were used in 113, 111,112 and some 108/109 cars. About 850 went into the 113 cars.
Daniel G Caron