I think that your 420 SEL should use ATF in the power steering unit. Check your owners manual to be sure. I don't think you should be using MB power steering fluid. I believe that was for later models. I changed the filter in my 300D and had just a little difficulty removing the thin friction fit fastener that held the filter in place. I just wiggled it off. Keep us posted on your success. My 1993 400SEL does use the MB power steering fluid.

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Richard Seaman
1982 300D Turbo 198k
1987 420SEL Sold
1993 400SEL 123k
MBCA Desert Stars