My XJ is NOT my DD.
Because it can sit long periods of time without starting i think 15W-XX has a better chance leaving a thicker film of oil and sticking to the pistons/rings/cam and not draining back into the oil pan as quickly as a 5W oil would.
Also with a quality oil filter with a good drain back valve, dry starts are minimized.
The instant my engine starts my oil psi starts to rise with the 15W-XX.

Right or wrong i will never feed my 4.0 5W-XX.
My newer vehicles? Yes because of mfg'ers power train warranties.
After the warranty periods end you guessed it..yep i'll bump up the viscosity to 10W-30/40.

By the way i've given up on the cooling thing until next Spring.
Got too many other BS things going on.
If needed, i'll just keep using my manual fan clutch set up in the mean time.

One Yureeka thing i did notice, the other day i decided to do some mild baha'ing down the ATV trails back in my woods.
I forgot all about the cooling thing and to my surprise while crawling down the trails, my efan kicked on a little past the 210 mark!
The only thing done was replacing the motor mounts and the fan clutch.
A fluke or some wire or connector got nudged during the replacement process or the fan clutch some how?

Also determined now with the brown dog motor mounts, my AC compressor is shot or less likely the PCM is not upping the idle speed.
Turning AC on at idle shakes the whole dam Jeep much worse now.
It never ends.

 

I don't blame you. It's frustrating me, and it isn't even my Jeep! I think we've damn near exhausted any ideas I have. Maybe it's best to just take another fresh look in a few months.

Interesting... That brings me to a question. Are you using OBD-II readings to monitor temperatures, or just your dash gauge. I do a little of both and, though my dash gauge works perfectly fine, I can tell you that it just doesn't have appropriate display accuracy to really know what's going on. It can't really show temps within better than about +/-10 degrees of accuracy.

A bluetooth OBD-II interface that you can use with a smart phone is really cheap and I recommend using one. This is the one I use: https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

 

Cluster gauge only.

Other than some spooky incidental fix with the motor mount replacement, only other thing i can think of it was a cool 80 deg. day.
Maybe not enough heat generated to snow ball the coolant temp.
And the Jeep was moving albeit at a snail's pace and not sitting still idling.

With the Brown Dog mounts (rubber bushings), now turning on the AC shakes the whole dam Jeep at idle. Think the AC compressor is about ready to seize solid.
My priority now is to get the AC compressor replaced or wait til next Spring.