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.
1992 XJ -- stock (RIP)
2000 XJ -- lots of basic mods. Sold to fellow forum member. I miss it.
2014 JK -- the project is just starting!
For Sale to Good Home---2000 XJ Factory Service Manual (Printed Version) PM for details.