Okay--this is right from the ZF S5-20 tech manual Ref-247.
"For the lubrication of the gear box use good quality oils of viscosity groups SAE 90 (or SAE 80 or ATF oils (type A. A). Oils which may cause corrosion (rust) on steel or bronze parts or hardening of the seal rings are not suitable. Hypoid oils are not recommended since they also can cause the above mentioned damages. Change oil first after 4,000 to 5,000klm, then every 16,000 to 20,000klm. Drain oil immediately after stopping the engine, i.e. when the oil is still warm and flows out easily. Every time you change the oil, clean the magnet of the exhaust screw. In case of any leakage, immediately chekc oil level, then ceck every 5,000klm. Good quality oils come with all the necessary additives. Do not add any special additives"
It is interesting that the fluid by the manual is not restricted to ATF. At one point in my ownership of my 230SL, my ZF box was filled with SAE 90 and it was not very happy with it. Very stiff shifting and some actual grinding of gears. My box was rebuild shortly after by ZF but I never asked if this may have been the cause of any damage that they found inside. Needless to say, it has only been filled with ATF ever since.
George