It's the resistance curve of the sending unit you need to match your gauge. A resistance substitution box makes it easy, but you can do it with a few resistors. Most oil pressure units range between 25-300 ohms, usually 25 being 0 pressure and 300 high.
I would start with 35Ω resistor at the gauge to ground and see what the gauge reads adjust resistance until gauge reads zero that will give you the low side of the curve. Next I would try a 300Ω and adjust until the gauge reads its highest oil pressure, that's the high side. Use the values to find a sending unit that closely matches.