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Originally Posted by Tezzating
...a 2000 Grand Cherokee Laredo, 6 cylinder + 4 speed auto & 4x4.
Serviced the diff's with synthetic 80W90...engine with synth Rotella oil...
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Did you use Shell Rotella T6 Full-Synthetic? I ask because I spent some time debating what oil to run in my own cast iron I-6 recently, a Ford 4.9, and as much as I've always liked Rotella in my diesel trucks, tractors, dozers, etc and a 1951 Dodge 235 I-6, I just couldn't bring myself to pour it into a motor that I plan to run as long as possible and get good mileage from. The only weight of T6 (full-synthetic) I've ever seen is 5W-40 and the only T5 (synthetic-blend) is 15W-40, neither of which are suitable for either of our old-school style engines, in my opinion, and that of Mother Mopar and Father Ford.
Chrysler specifies 10W-30 for your motor and Ford specifies 5W-20 for mine. I don't know your geography so a 5W might be appropriate a portion of the year if you're way up North but other than extreme winter weather (and really only for initial starting after prolonged dormancy in sub-zero conditions) a 5W is too light of an oil for these motors. Consider this a dual-damnation if you proceed with grille blocking. That being said, a 10W-30 and a 5W-30 will behave exactly the same when hot and a 5W-30 and a 5W-50 will behave exactly the same way when cold. (XW-Y means X cold and Y hot, neither impacts the other outside their respective temperature ranges)
That 5W would be good (second only to a 0W in low temp flow) in extreme cold, no doubt about it but that 40 would be detrimental to FE at anything above extreme cold so that oil and mpg just don't mix.
Chrysler deemed the low-temp protection of 10 weight oil sufficient for your motor (again extreme northern cold not-withstanding) and the high-temp protection of 30 weight oil sufficient. I can see improving on the cold protection as a benefit to minimize engine wear on wintry starts but using the heavier oil (40), which warms up slower, adds drag, and will cause an increase in oil temperature.
I've rambled on here to say this: use whatever oil you want but please be smart about it and share them smarts with me, cuz I'm genuinely interested in ecomodding an engine similar to yours. FYI: my research (BITOG, Motorcraft, Fordsix forum, FTE forums, etc) lead me to use 5W-30 synthetic but I think 5W-20 would work fine and that the 30 is due to the heavy-duty applications this engine was popular in, which differ from my light-duty uses. I also use Rislone ZDDP additive due to mine being a flat-tappet motor.