I run delo 15w40. 413k still going.

Stop that you're gonna wear your motor out with that naughty oil on cold starts, lol.

I've ran it for 60+k.

I was just messin' with you. I have 15/40 Rotella in mine right now. Your 413K miles speaks for itself.

Speaks for the oil... And my ability to keep things alive!!!!!! Lol

You've clearly done an outstanding job....see a lot of 200-300K XJ's but 413K is exceptional, imho.

Good for you!
You discovered that $50 oil change works as well as $3 bottle of ATF!
I suggest you switch to Amsoil, it is more expensive than Mobil1 and definitely works better!

I pay for the oil changes anyway. About $18 each. It's not as though adding ATF negates the need to change oil or reduces the cost.

Decades ago we used ATF to de-gunk Ford 302's at the dealership. It worked. But that was decades ago when motor oil stunk. 75K and those things were plugged. Solid.
The design of that engine really hasn't changed much, but they don't plug up anymore.. in fact
most properly designed engines that are correctly maintained don't routinely gunk up anymore...that is largely due to improvements in motor oil. ATF has been left in the dust of the 70's in terms of detergency.
What cleaning properties it exhibits are largely due to the fact it thins the oil...same as adding a slug of MMO. Thinner oil cleans better. Try rinsing your dishes with molasses Vs. water and you'll see what I mean.

I don't use Mobil 1 in my XJ, just pointed out that it does a nice job of cleaning over time. Even cleans the accumulated crud off your engine seals, allowing formerly hidden leaks to surface. It gets blamed for leaks that already existed but were merely plugged with crap.
I've never used Amsoil products and don't intend to. Don't like Amway either.
Or Tupperware for that matter, lol.

Jeep Engine Oil Viscosity
This was taken from pages 82 and 83 of the 1989 Chrysler/Jeep owners manual.
Selecting Engine Oil Viscosity
When changing or adding engine oil select the proper grade by using the chart on the following page. Select the grade that corresponds to the ambient temperature range you expect to encounter before your next oil change.
30*F to 100*F+ 20W-40 or 20W-50
0*F to 100*F+ 10W-30 or 10W-40
-20*F to 60*F 5W-30

You still won't give up on the ATF, huh?

There are engine flushes that can be used like Seafoam and other brands.

Like Radi most eloquently explained, ATF ain't what it used to be and oil ain't what it used to be.

But, we need something to use on a neglected motor that has significant build-up from infrequent oil changes.

I have had success with MMO and Rotella. Seemed to melt away the build-up over time.

I can't remember what it was that I used that one time.

It was some bottle off the shelf at O'reilly's. I ran it in a seriously sludged up Ranger 4.0 and it did OK. The engine was way past additives. I ended up pulling the whole thing apart.

Im running a half pint of seafoam right now. And in 1500 miles im gonna pull the valve cover off and see while comparing pictures. Then ill add mmo to my next oil change

No need to pull the valve cover. Just shine a light into the oil fill hole.

It is time to change oil.