Not trying to start a big debate on the difference between synthetic and conventional products, there's enough of those out there already. It's hard wading through all the internet junk and opinions when most of what I can find is so outdated.

My truck has the 3.73 rear (I was thinking of a re-gear but it won't be anytime soon now) and typically does 600 miles a week on the highway. I don't tow anything and every once in a while haul things, but usually only a couple hundred pounds.

I'm switching over to synthetic fluids and was finding only anecdotal evidence for leaving the rear gear fluid conventional. I'm only asking for your experiences with the two. Or links to something verifiable that I can read to help me make a final decision.

The other question is, beings my used truck didn't come with an owner's manual, what weight? I think it's (75 or 80)w/90 but what is the 75w/140 I'm seeing? Is that for just towing or off-road when the gears get really hot? Or for limited slip or posi-trac or locker? I don't have any of these. Just a plain old 3.73.

Thanks.

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Little Black Ranger: R.I.P Born-1997 Died-2018. 297k miles of good times. You will be missed.

Oh Deer: 2003 XL 2.3L 5-speed stick w/OD. Current mods: Body work after hitting a deer 12 hours after I bought it. Averaging 32 MPG on the highway.