The spec oil for the vehicle is 10w30. The manual, if you had one, would only suggest 5w30 for colder temps. Shear in 5w30 was more of a problem when the vehicle was built than it is today.
I have owned 3 of those in line Jeep sixes, and studied countless UOA's and had some of my own done as well. While the Jeep 4.0 will seem to run happily on almost anything, if you really want what is best for the engine you should choose a thick for grade 30, (such as Rotella 10w30 or Pennzoil HM 10w30) or a light for grade 40 (such as Castrol 0w40).
Most will tolerate thicker oils like 10w40 or even 15w40's, but that is overkill in anything but a badly worn engine IMO. The sweet spot is a thick for grade 30 or a light for grade 40.
If you must use 5w30, try to find a "Euro" oil that will have a higher HTHS.
Enjoy your Jeep!
I have owned 3 of those in line Jeep sixes, and studied countless UOA's and had some of my own done as well. While the Jeep 4.0 will seem to run happily on almost anything, if you really want what is best for the engine you should choose a thick for grade 30, (such as Rotella 10w30 or Pennzoil HM 10w30) or a light for grade 40 (such as Castrol 0w40).
Most will tolerate thicker oils like 10w40 or even 15w40's, but that is overkill in anything but a badly worn engine IMO. The sweet spot is a thick for grade 30 or a light for grade 40.
If you must use 5w30, try to find a "Euro" oil that will have a higher HTHS.
Enjoy your Jeep!