I would go for the sending unit. I had replaced the sending unit on my former 190D once and this renewed little rascal gave a reading of 0 every time I got of the autobahn after driving at 140+km/h speeds.
The first time it scared the hell out of me. I opened the windows to listen to the engine, but it definitely sounded like it was still enjoying a good oil pressure.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)