MB provides fluid specific for power steering. I used ATF in my power steering for a long time and the pump seal eventually failed.
I read somewhere a year or two ago, that ATF has changed over the years, and that it was no longer good to use in power steering. Wherever I read this, it said that ATF would cause harm to power steering seals.
If you don't want to get the fluid from Benz, you can get generic power steering fluid at any auto supply store. This is what I now use, and have had no more problems.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in