Some would argue that you need the MB power steering fluid designated for your 420SEL(and other 124/126 cars).
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=24B16YOHO24Y11LEU2&year=1991&make=MB&model=420-SEL-001&category=M&part=P%2FS+Fluid
This is what I use, but I'm inclined to agree with Brewtoo. The look, feel; smell, etc. of this MB fluid is VERY similar to plain-jane PS fluid. Now I realize that look/feel/smell ain't a scientific observation; I just can't help but wonder if everyday PS fluid would work just as well?