Can be the oil psi sending unit as well..
I have replaced the sender on my 190D by a new MB sender once. After about 3 months and a trip of 100km flat out my gauge went down to 0 while I stood in front of a traffic light (can you imagine the look on my face...).
I turned down the window to hear the engine, but it still sounded like the oil pressure was good.
It took me a new oil-pump and oil-filter housing before I found out that the sending unit was BAD. Normally when the sender is broken or the cable is disconnected (partly or total) the gauge will go directly to 3 when the ignition key is turned on.
with a minor 600+k