Something is off by at least 25 psi, since you have zero oil pressure when your engine is not running. It could be your gage, or it could be the oil pressure transducer/sender. Best shake down is to pull the sender and install a mechanical oil pressure gage and get an accurate read. Normal hot oil pressure should be 5 psi min at idle, and increase with rpm.
If the mechanical gage says all is OK, check wire connections at the sender for corrosion. Still bad, check the wire connections at the dash gage. Check the ground wire at the gage also. If still bad start with a new sender. It's less expensive and less hassle than installing a new dash gage. Finally, new dash gage, but I think you won't need one.
