Oil pressure light when idle?

I am using 30w for high mileage engines.

Also the oil pressure switch was replaced when I had the engine rebuilt.

Does anyone have more inifo on how to install a mechanical oil pressure gauge?
Do they make them to just replace my current screw in switch?

I already have a gas gauge and temperature gauge hangin from the dash. not sure where a 3rd gauge would fit.

any help is appreciated.

also I do know they put in a new oil pump when they rebuilt engine.