Generally if the resistor was in the wire it wouldn't be in the gauge cluster. If it is in the cluster, all I did was take a piece of wire and soldered it on the connections the resistor used and left the resistor there so you can go back to stock if you wanted easily. I did because I installed an after market.
As for readings, your best to get a real oil pressure gauge installed to give you psi then look at what the stocker gives you. Anyways, for me hot idle gave me around the N which is around 25psi at 600rpm's and cruise gave me a reading around M which is around 65psi at 1500rpm's... according to my aftermarket gauge, and thats with Mobile 1 10W30. Thats in my car, but results do vary from car to car.
Its is HIGHLY recommended to install an aftermarket to be sure.