There is not a definitive answer. From the factory our cars had ATF in the power steering pump. Since then Mercedes has switched to using power steering fluid in their pumps. Some state the additives in modern ATF are not good for our pumps.

I use ATF in mine. I don't think it is too big of a deal which ever you want to use. If you choose to switch to power steering fluid I would completely drain the pump and steering box and flush them otherwise I would stick with ATF.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD