RM92400E and sschweg,
The oil pressure circuit will read 3 when the circuit is open and 0 if grounded.
I have had this problem on my 400e. It looks like there is a flaw in a check valve located in the filter housing. I believe this is the same valve that causes the timing chain rattle due to a oil leak down.
You can verify the operation of your oil pressure gauge by simply removing the connector on the sender and ground it. The pressure gauge will then read 0.
I have found that the problem happens when I use dino oil. If I run synthetic, the problem does not happen.
Here is something you can try... it worked for me.
Drain the oil. Remove the oil pressure sender. Remove the oil filter and pour 1/2 to 1 quart of fresh Mobile 1 into the filter housing. This will flush out all of the old dirty oil left in the filter housing. Pour until you get clean oil flowing from the oil sender units connection.
Reconnect the oil sender and new filter and fill with Mobil1 synthetic. I use 10-30, but since we are getting into winter, you may want to try the 5-?? Mobile 1 synthetic.
When you turn on your car, you will see that the pressure gauge is at 0. When you start the car, the gauge will jump to 3, it may take a few minutes for the oil to get warm. After the oil warms up, the gauge will show 1.5 to 2.5 at idle.