A lot of answers here and it's clear that nobody understands oil.
If you want the short answer: in your Jeep engine 4-cyl or inline 6
run any API certified oil in the correct weight (5w30 winter, or 10w30 summer). The only caveat I've found is that synthetic does flow better in winter — so I'd recommend that.
Now for the long answer:
All synthetic oil is NOT made the same. All of the major brands except for Amsoil and Mobil 1 are conventional oils (ie. petroleum base stock) that have been filtered so that all of the molecules are closer in size than a normal conventional oil is. By filtering the conventional oil to turn it into a synthetic like that does make the oil much more consistent in how it behaves at temperatures and improves lubricity.
Amsoil and Mobil 1 start with a synthetic base stock that they further blend and add additives to. By starting with a synthetic base stock they are able to physically put more additives into the oil and end up with the same lubricity and further control how the oil behaves at certain temps. What helped me wrap my mind around this best is by comparing typical 2-stroke oil on the store shelf with Amsoil "Saber" 2-stroke oil: it becomes clear that you can physically run Amsoil at a 100:1 mix and receive the same lubricity as typical 2-stroke oil mixed at 50:1.
So now that we understand why Amsoil and Mobil 1 are superior oils, why did I say it doesn't matter in the short answer? Because our engines don't get super hot like highly modified racing engines, they don't rev very high, they have relatively high tolerances which allows oil to flow easily, and they don't have a high internal pressure. After a few years of testing my own Jeep engines I haven't found any benefits to any certain oil even when sending the oil in for analysis.
The only caveat I've found is that synthetic does flow better in winter — so I'd recommend that.
The best way to test this is to buy a quart of each before your next oil change and leave them outside overnight on a cold night. Pour both into your engine and observe how quickly they flow out of the bottle. Around here a conventional 10w30 will flow like honey and a synthetic 5w30 will flow out smooth and quick. Inside your engine that equals how quickly your engine's moving parts are adequately lubricated which will reduce metal on metal wear.