I've always used T6 because it's cheap, haha...

 

Ok...
what do y'all think of a 15W-40 T6 in our old 4.0s?

Any glaring issues with that weight in a pretty mild climate? I can get my mitts on some of that.
I had been using the 5w 40 T6 for the last 5ish years in it without any issues. At least that I know of.
Little bit of lifter tick usually on a cold start but that's about it.

 

It's all I use. 15/40 in everything. BUY IT!!!

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

 

Done cruiser...15/40 it is. They had 2.5 gallon jugs on for $55. And at least around these parts...that's. Good deal. Now I'm collecting fluid to change for the Trans, transfer case, and brakes. Ive gotta do it in one pass while my wife is out of town otherwise my time gets sucked up with the Ole "honey-do" list. We all know how that goes. Lol...and the jeep needs some love