It's nearly always the speedometer cable. I just used white lithium after cleaning the cable. There are cable specific greases if you want to do some searching. Take care to not apply too much and don't force any into the speedometer housing.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120