It is true that ATF has changed since the time these older Mercedes were manufactured. It was once a fine substitute but not anymore. You're basically guaranteed future issues if you use it exclusively. I use only synthetic power steering fluid in all of my old Benzes. Pentosin and Febi/Bilstein both make good ones. They range in color from a clearish fluid, to light amber/yellow to a greenish color.

The seals, hoses and steering box will last a lot longer with this type of fluid over modern ATF.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles