Im not having faith in the OEM gauge so I hooked up a aftermarket mechanical to hopefully diagnose a faulty sending unit. a couple times the OEM bottomed out and kicked the low pressure light on, after installing the aftermarket gauge, it reads virtually the same except it didnt drop below 8-10psi at idle like the OEM did. other readings show 28-30psi at 2000 RPMs, 20-23psi at 1750 RPMs.
Any thoughts on the matter?
Also fwiw, when I done a oil change, I read these 4.0s have a 6qt capacity but it only took 5.5qts with filter change to bring it up to full, Im thinking/hoping gunked up with a failing oil pump, this is a 2000 XJ so you see my concerns....thanks

 

The rule of thumb is 10 psi of oil pressure for every 1000rpm and the lowest at idle is 13psi. You're fine.

 

x2

Oil pressure drops with mileage due to bearing clearances opening up. Although a higher flowing oil pump may produce a higher residual pressure, it is not likely the oil pump at fault for your lower oil pressure.

 

The oil pressure spec for the 4.0 is as follows:

*Not less than 13 psi at HOT idle
*Between 37-75 psi over 1600 rpms

If you are seeing verified oil pressure of around 8 psi at hot idle which from your post it sounds like you are, that indicates a problem with a "clearance" somewhere in the engine. Could be an oil pump, but a more likely suspect is the engine bearings.

With an 00-01, you have the infamous 0331 cylinder head, which has a bad habit of cracking between cylinders 3-4 on some unlucky vehicles.

The #1 symptom of a cracked 0331 head is "unexplained coolant loss". You need to be watching the level of coolant in your coolant reservoir like a hawk. Are you losing any coolant?

 

If the gauge starts acting funny, replace the oil pressure sensor. They fail all the time and cost like $12