No, synthetics do not ruin the rear main seal. Where did you hear that?
What's the history on this vehicle?
How much do you want to spend?
Using the information provided, if it were mine, I'd go over to Wal-Mart and get:
Mobil Delvac 1300 15w-40
The Mobil-1 sounds like a great choice. It seems like that engine prefers a heavier 30wt or even 40wt oil. If you are interested in High Mileage oils, Castrol HM is a heavier 30wt, and has shown great UOA results. If the rear main seal is an issue, it has seal swelling agents to aid in the repairs. Good Luck! And yes, Motorcraft seems to have a great line of filters!
I have a 1996 Cherokee with 130,000 on it that has run M1 10w30 since day one. The motor uses no oil whatsoever, and runs like it did the day it was new, with absolutely no leaks. If you want longer OCI's then I would use M1, if you want to change it more often then use a good dino 10W30.
Korry do a search on "Jeep" and/or "4.0" and you'll see what others are using and you'll even see lab test results from used samples (UOAs).
This engine has come up dozens of times before.
--- Bror Jace
I'll throw in my experience too.
My 96 Cherokee has always run on 10W30 dino. Castrol GTX for the first 5 yerars, the I switched to Chevron Supreme, not because anything was wrong with the Castrol, just read good things about Chevron here, and thought to try it.
My engine doesn't consume or leak any noticeable amounts of oil. I'm thinkin gof trying one of the high mileage versions of these two oils soon.
I always use eithe Napa Gold or Purolator filters.
I have a 98, and Have been running synthetic for 15,000, running 10w40 syntec to try now, I have a small seepage at the rear main, however I have not had any noticable loss in oil and only add barely 1/4 of a quart over 5000 miles. And the seepage seems to actually be slowing some now, I can only assume that it is the synthetic swelling the seal some.
My work vehicle is a 95 Cherokee with the 4.0. It is pm only twice a year (every 6 months) and the oil used is Exxon XD-3 15w-40 with Fram filters. It currently has 206,000 miles on it and won't be replaced until next year. It hasn't had an easy life with lots of extened idling and stop and go driving but it is still a strong motor that still gets it done without showing any signs of quitting. So my vote goes to using a heavy duty 15w-40.
ran my 1995 4L jeep engine to 155,000 miles on Mobil 1 10w30 + PureOne oil filter every 5,000 miles. Sold as no leaks and no added oil between 5,000 mile changes. Started Mobil 1 at the first 2000 mile change from OEM oil.
for the past 12,000 miles i have been using Valvoline's Maxlife 10w30 and well pleased with it...
now, since hearing that the new Penzoil HM oil is a lil thicker i opted to get 6 qts of the HM and the oversized Wix filter for the 4.0
whats the deal with the Motorcraft Fl-1a? filter...
isnt the wix, napa and purolator oversized just as good??
oh, i forgot to add that i have 174,600 miles on the ticker on it
CLEAN IT OUT!
Recommend that you use Auto-Rx (per directions; see website under Forum Sponsors) along with an inexpensive 10W-30 and filter. (And then every 25-30k).
KEEP IT CLEAN!
Also recommend that you use FUEL POWER to help clean up, and maintain fuel system/combustion chamber. (Website; as above.)
If you'll do a search on my member number in both "Additives" and in "Used Oil Analysis" you'll see the experience I've with both on our 2001 JEEP Cherokee (now at 77k).
Have used these products with excellent results (the FP pays for itself just in out-of-pocket fuel costs; and I've had  Jeep techs tell me that rarely have they seen such good numbers when checking over the fuel system).
As oil, I have used Mobil One 10W-30 since break-in, switched for 16k to Mobil One 0W-40 (see oil analysis), and am now using REDLINE 10W-30.
And, while the FL1-A size is the cross-reference, I'd suggest the better BALDWIN B2-HPG, the FLEETGUARD FL-3487 or the DONALDSON P1690781 (ck part numbers on both). These are "Extended-Life, Severe Service" with better flow and service life.
(Of course, if you are going to run an ordinary oil only 5-6k miles, then an WIX 51515 is a very good choice (same as the NAPA Gold).
You ought also to search Member #9 (Terry Dyson, of DYSON ANALYSIS) recommendation of these products, along with LUBE CONTROL. (Please understand that there are many excellent commentators here, I simply have used Terrys' services in the attempt to keep my motor as new -- not simply "ok" -- for better than 150k).
Do some searching. As Bror Jace indicated, this motor is well-represented around here.