I read not too long ago that modern ATF's are not good for the seals in you power steering pump.
I believe this because the seal went out in my pump after changing the original fluid and using ATF. I would reccomend using power steering fluid from the auto parts store. It's worked good for me with my new pump.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in