As mentioned above, the oil pressure sender is electric and is a known failure item. Take the oil filler off with the engine running and look for oil slopping around in there. If oil is moving around in the rocker cover, you have oil pressure and know to look at the sender unit.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]