I would run regular 87 octane fuel, and see if you have any pinging.
Octane numbers are "heat" ratings, not energy ratings, so a gallon of 87 octane and a gallon of 91 octane have the same energy, so same MPG.
The higher octane just has a higher self-ignition heat rating, so 87 octane will self-ignite at a lower temp than 91 octane.
On lower compression engines(like the 4.0l at 9:1) 87 octane should run fine with no pinging, when an engine is at 10:1 and up compression, 87 octane would ping.
Carbon build up on the piston top and head reduces the size of the chamber so compression ratio changes, it goes up, so pinging with lower octane number.
Often regular gas has ethanol or similar added(10%), this actually raises the octane(self-ignition temp) but reduces the energy per gallon, but not by much, maybe 2%-3%.
91 octane fuel often has additives for cleaning injectors, IMO, the price per gallon isn't worth it when you can add a can of Seafoam to the tank once a year($8) and get the same results.
Octane numbers have nothing to do with "burning cleaner" either, if you were to run two similar engines, one on 87 octane and the other on 91 octane for 50,000 miles then pulled off the heads, the cylinders would look the same, all other things being equal.
NOx numbers are really only an air quality concern, EGR systems were not added for any engine function, non-EGR engines were simply not required to meet the lower NOx numbers where they were first sold.
However......EGR systems can improve MPG when you add computer control with EFI, O2 sensors and DIS(distributorless ignition), this is because a leaner mix can be used under load without causing pinging when exhaust gas is added to the intake air.
Running a richer mix lowers cylinder temps, retarding the timing lowers cylinder temps.
The 4.0l uses DIS so spark timing is not adjustable without computer changes.
Same with EFI system, no real control over fuel/air mix, although you can sometimes highjack the ECT sensor to tell the computer the engine is cold
.
This will cause computer to run a richer mix with advanced timing.
The oil condition doesn't really tell you cylinder condition, carbon build up in the cylinders is on the piston top and head, so not in places oil should be
Carbon build up comes from engine running rich over a long period, or constant misfires leaving unburned fuel in the cylinders, even cheap oil that vaporizes and is sucked into the engine via the PCV valve.
Dirty injectors drip fuel instead of spraying fuel so all fuel isn't burned.
Spark plugs that have been in a long time can tell you about carbon build up, because it builds up on the spark plug end as well.
There are methods to clean out carbon build up in cylinders, without removing the heads, but I have never tried them, others here have.
Compression test can often tell you about carbon build up, but certainly not a definitive test, 4.0l should have compression numbers no higher than 180 psi, if you see numbers higher than that then there is significant carbon build up.
Lower fuel pressure can lean out fuel mix and at higher rpm raising cylinder temp and NOx.
DPFE sensor, if you have an EGR system, will be Black and have 2 vacuum hoses, one going to the EGR valve, and an electrical connector going to the computer.
The computer opens the valve inside this to pass vacuum to the EGR Valve to open it.
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