I have a 1997 XJ with 150,000 miles on it. Its had a wonky oil pressure sensor since I got it ( the guy I bought it from said he repalced it but I dont think its OEM). When I first got it, the gauge would only go up with my rpms and read that it had no pressure when idiling, Ive never had any problems or leaks so I just let it be until now. I changed the oil recently using a Mobil1 Filter and synthetic oil and ever since, the gauge will barely move no matter how many rpms. Ive checked it many times to confirm im still holding oil and the engine sounds normal. Im wondering if anyone else has had this problem with synthetic in older engines? Maybe I should just stick with conventional. I thought I might of broke the sensor when I was changing the filter but it intermittently works so im not sure if thats the case.

My personal view is that even when it was new an XJ motor was too old to run on even semi-synthetic, which was meant for much closer tolerances - & 150,000 miles certainly won't have improved matters.

That said, that is not your issue. Replace the sender if you're not happy about it, they're cheap enough but as the fault's intermittent, I'd suspect the gauge, assuming the wiring looks OK.

First thing I would do is clean and tighten the plug to get a better connection. They get dirty and stretch out. I used a tiny flat blade screwdriver. That took care of the same problem on mine and been working well for several years now.

It originally started right after an oil change because I disturbed the loose connector when accessing the oil filter.

Hello
This engine has old technology , first bypass the or remove the oil sensor and take a direct reading with a handheld oil press guage to see if
you might have a defective oil sensor. ck pressure at 700rpm then 1500 then 2200 and see if your pressure is linear.
Its a good chance that your worn engine cannot handel fake oil and your piston bearings are worn. Change your oil to pennz oil with mopar
oil filter and try again. The longer you wait may harm your engine due to low pressure providing that your oil sensor is good.