i have seen a handfull of questions like this in forums but im still unsure of what is most likely the issue.
my oil pressure has started to go to 0 only when idle at operating temps or hotter as i rev/drive it will go up. i have changed to oil to rotella 15w-40 from 10w-30 mobil 1 but no work changed oil filter while at it (mobil1 filter) got a new oil sensor still no good.
i hooked up a mechanical gauge cheap one from wallmart and still same. i have seen that it could either be oil pump or bearings. can a faulty oil pump cause this? or if it was a bad oil pump would it just give me 0psi no matter what?
edit- i dont hear any bad noises from engine 01 xj 2wd

 

If the pump puts out reasonable pressure at revs, its more likely to be bearings

You could have a partially blocked filter, or bypass valve (if the XJ has one)

In your case, I would send an oil sample for testing cost about $50, shows presence of many things, including various metals associated with excessive bearing wear

 

I would be curious to the year cause of the 0331 cracked head issue in '00 or '01.
No oil pressure at a very hot idle being a classic symptom.
Oil sample test would not be a bad idea.
But first I would see if the hole the sensor/tester goes in is all gunked up.
Guys have reported this happening and IIRC use a pipe cleaner to clean it.

 

it gets around 15-35 psi driving at operating temps

 

i did a suggestion someone had for that issue idk if info help but i disconected sensor and coil connection and started it up and oil shot out.
EDIT: used pipe cleaner method and it was good i had it idle till operating temp/a bit hotter then drove it a bit. was about 20-40 psi and idle was around 8-10 psi. let it idle some more and same turned off car then started it back up after a couple min and back to 0. rev and go up but then back down after stopping