If the seller has done what he claims, and the hot idle oil pressure is showing "0", then it's time to temporarily install a mechanical guage on the engine, and get a reading on the REAL oil pressure.
If it's as low as the electric guage says it is, then the problem is probably loose bearing clearances, and, at a minimum, a new bearing set will be needed.
I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
So...I stick around and give advice.