OK so here's the deal... bought a 2000 xj about a month ago... the oil pump had recently been replaced. When the engine is cold no problems, however once it is completely warmed up, when I come to a stop the oil pressure gauge slowly drops - understandable... but if I sit for a few seconds it suddenly drops right to "0" and the check guages light comes on. First off.. how can I check what the actual oil pressure is, to rule out a bad gauge or connection. 2nd... what the heck else could this be? This thing has so many stupid little electrical problems it's unreal.

 

If you have a bad gauge then simple buy another one.

Thats the only way to tell you the truth

 

Just Expect Electrical Problems

:P

But seriously, I would change the oil and put in some good 10-40 first and see if that helps. Around here Auto Zone has a 5.25 q jug of GTX high miliage, which is a semi-synthetic and a free filter for $18. Walmart also has the same oil for $14.50.

mine starts off cold at around 50 then drops to 30 when moving once warmed up, it goes down to 20 when stopped for a bit, after changing the oil with GTX 10-40. When I first got it it would be around 20 driving and drop to almost 0 when stopped.

 

2x on the oil. I had 10 w30 conventional and I had low pressure as well. Mine would drop to around 10 after warmed up. I put the 10 w40 synth/blend. Valvoline Max life high mileage and I added a quart of lucas heavy duty. My oil pressure went up and my engine idles much better as well. Of course it has only been a day but I will try to remember to update you in about a week.

 

good chance it is the pressure sending unit. It sits right next to the oil filter and if it has ever been to a quick lube joint - I bet they banged it and messed it up. Had it happen a few times to mine - now I just do my own oil changes ALWAYS (no garage so winter I would take it in).

 

pull out the sending unit and install a mechanical tester with a gauge similar to a compression tester to know for sure

 

+10, that's the correct way to check if there's a oil pressure problem. Filling an engine with thicker oil just moves power from the drive wheels to the oil pump due to having to move a thicker fluid with more resistance to flow and higher oil temp. More flow equals lubrication, not more pressure. A good rule-of-thumb is 10 psi oil pressure/1000 rpm.

 

X3 ...verify with mechanical guage and go from there.
Possible with a 2000 that had the 0331 head that it has bearing damage from water in the oil and PO tried to patch with new oil pump.
Does the head look like it has been off ? Does it look milky under the oil fill cap ?