When you pull of the cover, the level should be at the rim which is about 1/3" beow the lip. For fluid, your owner's manual should tell you, and it probably says to use ATF. It's not until the last couple model years that MB specified their "special" P/S fluid. Remember, there is a filter in there that needs to be changed at regular intervals, along with the fluid.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member