I had an independent Mercedes-Benz mechanic do this service only to fine out he used ATF transmission fluid. I checked with Mercedes-Benz sources and was informed that it would be OK, but that they recommened factory Fluid for the system. I had a good friend of mine help me purge the ATF fluid completelyby disconnecting the return line hose and using a small bucket to catch fluid from the system while pooring in Mercedes-Benz power steering fluid until it ran clear. Then put a new filter in and toped off to the correct line.
I am much more satified nowing if that this service was done properly even though I do not drive the car much and it has low milage under 40,000 miles.
I beleive following recommended service will help protect the mechnical parts of this car.
Strangely I do not see any recommendations for changing Power steering fluid in the Mercedes-Benz service guide for my car 1992 300-E.
Bob Geco