a fram filter collapsed internally and stole all my oil pressure at idle a bunch of years ago. Didn't realize what the issue was (I feared some sort of internal engine failure but it made mo noises and didn't leak any oil) and was on a trip so I just kinda dealt with it and 3-pedaled it ti always keep the idle up a bit for a few days til I got home. once I changed the filter all was magically well again.
arguably the HO harness is more reliable since it includes scores of updates and upgrades over the AMC wiring harness.
But the first step is seeing what you've got to work with. keep us posted
sounds like part of your issue could be a bad ground at the drivers side taillight. the fuel pump and the brake lights ground to the same spot back there.
also I moved your topic to Tech so more eyes will see it.
what you have now will surely dictate the easiest course of action. the HO swap is pretty straightforward. remove everything from MJ, install everything from XJ. there are some hiccups, but nothing that hasn't been tackled many times before.
10w30 full synthetic is the new guideline from Chrysler, but the old oil spec will also work as long as it's not the synchro eating GL-5.
where's that update?