Also, keep in mind that minimum 13 psi is usually with 10-30 oil, which is what jeep recommends. I used to have about 16-17psi with Mobil 1 10-30 full synthetic. I switched for 5-30 Mobil 1 extended performance (the "gold" also full synthetic) and my pressure is now about 13 when oil is over 200 degrees. Here is the thing: the 5w-30 I use now has a lower viscosity than the previous 10w-30 oil I had, this based on the spec sheet I got from mobiloil.com, so less pressure is something I've expected. I wouldn't worry about this, there is a direct correlation between flow and pressure, with less pressure there is more flow, so pressure by itself is not everything, a proper combination of flow and pressure is what makes the engine happy. Also, depends on how many miles and how worn is the engine, this is another reason many "high mileage" oils have higher viscosity... They try to compensate the engine wear (journals, bearings, etc). So same brand and same weight oils one "normal" one high mileage may have different viscosity, with high mileage having more than the "normal" one. But this just hides real issues, so I'd just use normal oil, synthetic if it's within the budget and when pressure issues arise, fix the real problem, don't change the oil to thicker just to trick the gauge to show more. The "old school" oil pressure recommendations are 10psi of pressure for every thousand rpm. Many folks here on the forum go with 40 weight oil mainly due to the fact that our 4.0 engine runs a bit hot, this due to relatively small radiator and lack of an oil cooler, it's not uncommon to see 220+ oil temperatures even when not off-roading, just city driving. When the oil reaches over 220, it tends to become even thinner, so that's the reason of choosing a higher weight, thicker oil. The proper way though would be to put an oil cooler to the jeep, but that's a pain in the butt since the front of the car is relatively packed with the existing radiator and maybe custom trans cooler, and there is no place to mount it on the engine either (best spot would be under the front bumper, which is exposed to potential debris). To make a long story short, I would agree with everybody else, don't worry, pressure looks good, just be religious with oil changes and this 4.0 engine would last till you retire, lol.
Edit: just saw you said the gauge fluctuates, first, I'd check the gauge connectors to be clean and tied. If it fluctuates slightly (i.e. Not jumping all over) you may have some sludge from improper, too many miles interval oil changes, best would be to take a look at your old oil that you just changed, you'll see the sludge in there if any. It could be also a faulty gauge, which is relatively easy to change.
