My 1989 xj 40 oil pressure gauge 3.6 is starting to show zero psi when hot and at idle. Level is fine so probably the sender. I can’t seem to find a true pressure sender instead of an idiot light, anyone have a part number for a replacement? I like actual pressure as you can keep an eye on potential problems
You can get a Wells or Beck-Arnley aftermaket one from Rockauto but may need to make up a 75ohm resistor to sync up with the dash gauge. Either that or go resitorless and brag how your oil pressure is so good it redlines. 
I dont want an idiot light type set up! Maybe something from a land rover might fit? My 94 ram has an actual gauge as does my wifes 90 miata, there should be something that would work and show actual pressure readings!
Those are not idiot lights - they are transducers, just like you have fitted right now and read actual pressures. I have a beck-arnley one myself. You’ll still need to make up a resistor though, but that’s no biggie.
Here’s the URO one on Rockauto, $18
Do I need to ad resistors, or is it plug and go?
One way to find out… Fit the thing and see if the dash gauge makes sense- if not, make up a resistor. It’ll cost you another couple of bucks.
I believe there was a GM one that works as GM trucks and some cars had gauges that read real pressures. And there is always Nissonger the Smiths instrument agents located in New York. They may be able to supply one but not sure if the XJ40 instruments were Smiths equipment or not.