This might be a time for your favorite indy to inspect and confirm current status.
On another German car, this vid notes a physical surface difference between a TRW and ZF rack.
Don't know if it applies anymore.
Might help, if, as GB says, a TRW replacement was used in a repair and that TRW needs ATF (not clear if it does, since it would be odd to require a part sold as OEM replacement use a completely different fluid).
Of course, it's possible that someone added the wrong fluid to a ZF, as well.
If it is a ZF, yes you should drain it and replace. Use the newtis at their discussion of pump and rack replacement to check best line to open to drain the fluid.
Also, the reservoir has a filter in it that gets restricted over long periods of old fluid. Cheap replacement, so do it when you do the fluid. $25 or so.