Service Manual
Chassis and Body
Series 124, Volume 1 (covers 1986 - 1992)

Job No 46-130 (Checking oil pressure of power steering pump)

Oil Types/Filling capacities and oil level checkup
Steering Gear Oil (ATF)

Obviously, you cannot go wrong with the Mercedes PS fluid.

Personally, I have used Mobile 1 synthetic ATF in my 1990 (sold) and 1991 300E now.