Agree on the valve tick in the second video
15W-40 would be thicker, tre first number refer to thickness when cold, the lower number the colder weather it will still perform at start ups, hence why Alaska ppl use 0W-???
The second number is "thickness" at operating temp, you SHOULD stay within manufacture specs for your climate, then SHOULD be ok,,,
Idk why, but the 4.0 seems picky at oil, and might call for something ells than what manufacture listed, especially when they Get older and more worn.
We tried 10W-40 Dino, 10W-30 semi syn, 15W-40 Dino, in my friends 4.0 ZJ, just because we had it around in big quantas and wanted to flush out old motor.
Deff runs smoothest and most quiet on 15w-40 and if you aint going below freezing it wouldnt hurt anything to try, but thats really up to you to deside.
Michael, oil and or flushes Are great things and MIGHT solve SOME issues, but will never Fix real mechanicly issues, look at my engine tick, i tried about EVERYTHING, parts NEEDS to be replaced.
Do seafoam, do "ATF flush", try another oil,
THEN you will know, BTW its easy to pull valve cover and have a look,,,