I've run 10W-40 in my M103 for the 20 yrs. I've owned it. Apparently working just find.

My MB has been inactive until just recently. About a month ago I filled the gas tank up to the top for the first time in awhile. The fuel gauge went up about 3/4 of the way. A short while later it kicked itself up to full.

Maybe you sender needs replacing. Maybe the elect. connections simply need cleaning. Inactivity breeds resistance in elect. circuits.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"