The manual does specify synthetic. Syn generally does also have a longer service life and can improve mpg. However, tests have shown gears run cooler in dino, but this isn't likely much of a concern in our jeeps anyway as we aren't exactly pulling super heavy loads and such.
Basically, for me, I haven't seen much of a difference in mpg to justify the cost difference, especially considering it gets changed so often. It's getting changed due to possible contamination instead of the oil beyond its service life. I run syn in my engine and t-case and trans whenever it gets changed, but dino in the diffs because it gets changed far too often.
For what it's worth, I've spent my share of time in deep water and never did I have a noticable issue of water intrusion afterwards, but it's too costly to just hope for the best when I could spend 25 bucks and changed both axles fluids in the parking lot before heading home in about 20 minutes.
If I recall, the 75-140 is for the rear axle and the front is 75-90- or 80-90 or something like that. In the summer I will put the 75-140 in the rear, but I've never had an issue running the 75-90 or 80-90 in both in the winters either. Honestly, unless you're pulling trailers a bunch, you could likely run 80-90 year round, but I do my share of trailer pulling so I put the heavier oil in in the summer.
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Last edited by oz97tj; 01-09-2012 at 02:40 PM.