New Oil Pressure Sender
You'll have to change the oil pressure sender, to one that is compatible with your guage.
The OEM sender is only a 12V switch. No oil pressure = the switch is open. Oil Pressure = the switch is closed, & the OEM guage moves to "somewhere". So the OEM sender only supplys voltage/no voltage. It is not a variable signal.
There is a mod that you can do which will allow your OEM oil pressure guage to react like a regular guage. It involves changing the sender, and bypassing a resistor. However, this mod will NOT tell you how much oil pressure you have. It will only show when it varies.