I'm have a oil pressure problem with my 1990 Cummings. I have changed for the sending unit and have change from synthetic oil to 15w40 . The oil pressure will drop hot or cold and have but in a real gauge and the oil pressure is good . I need help to turn off the chime on the dash or to fix the gauge .

 

So with a mechanical gauge the pressure is ok?

Clean your ground wires. Where they attach to the frame and the cab.

Look for grounds under the dash also.