Don't assume the worst until you've eliminated all else. Disconnect the wire on the sender, test the gauge. If that's OK, then you change the sender (cheap compared to what an oil pump would be). I don't remember if its voltage or ground you use to test the gauge, but that's why service manuals and forums exist.

I have not experienced any strange behavior due to syn. oil. They're close enough that I don't think a mechanical sensor would care.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon