I just started using Havolene PS fluid in my 1982 and 1987 300Ds, and this was due to emptying out the reservoir and changing the PS filters on both cars. The 1982 may have had the original PS filter in it, and the fluid was surely nasty.
The Havolene PS fluid specifically lists "Mercedes" on the label. It is a clear fluid and reminds me much more of hydraulic fluid than ATF. I think it has a higher viscosity than ATF too.
The only easy way I've found to change out PS fluid is to pull the reservoir dry with a plastic syringe and tube. This probably changes out 1/3 of the capacity at a time. There's no drain plug on the steering box is there?