You can drain the PS from the steering box and get a clean flush very easily we discuss this on this thread: A few W126 repair questions
hope it helps. By the way I have used both ATF and PS fluid in older cars. I concur with LArry if you have the choice use the PS fluid it is not that expensive and was made for this purpose.
m

__________________
Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)

Mercedes approved only two types of ATF fluid for power-steering use: Dextron Type II and Type "F".

Most ATF fluid today is Dextron Type III which contain slipperiness additives, which is why they will eventually leak if you continue to use a Dextron Type III ATF fluid in your power-steering unit.

I myself just purchase MB's powersteering fluid, and no worries.

:-) neil
1988 360TE AMG
1993 500E