I think your car probably has an oil sending unit on the back of
the oil filter housing, and a wire to an electronic gauge. At least my 1987 W124 has this, so yours probably does too.
You do need to work on this problem fairly quickly, because if the oil pressure reading is accurate, you have an oil pressure problem that needs to be fixed to avoid engine damage.
However, it is a little more likely that you have a sender that is going bad. You should be able to select one on FastLane and install it as a test for about $60, if you want to risk that amount as a "blind" part substitution effort. Besides, you would spend that much if you just walk into a Mercedes dealership and say "Hello" to the service manager.
Give us some more information on your symptoms. In addition to what PEH asks:
What is your oil pressure reading when cold at idle?
What is your oil pressure reading when hot at idle?
What is your oil pressure reading when hot at 50 mph or so?
I had a sending unit bad on my 1987 300D when I got it, but with a different indication than yours. In my case, the oil pressure gauge would peg immediately when the key was turned to ON, even with the engine not running. This was a clear indication of gauge malfunction. Your issue is not so clear cut.
But you need to do something right away.
Ken300D