MB did fill these car steerings with ATF, dexron II to be exact. No issue in using dexron III on there. Be wary that it will permanently tint the system red.

You can use generic PSF, but it will be thicker to pump when cold. Orielly's sell a PSF which is close to ATF viscosity - its called masterpro PSF.

The steering oil you buy from mercedes benz is actually ATF DII. its also sold by LubroMoly in 1 liter packaging, Its most probably undyed ATF.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)