rear diff fluid

I have a 91 limited, 4.0, d35 with a factory limited slip. I have only had the truck a couple months, and checked the diff fluid over the weekend while I had it on a lift. The diff was way overfilled with what looked and smelled like ATF. Did the previous owner know something I don't, or should it be gear lube in there? The gears don't whine or grind.
Thanks

does it not have a towing package or something cause i have a 91 limited with a c8.25 with posi. yay 91 limited leather and power seats yay!!

76_iron said:

I have a 91 limited, 4.0, d35 with a factory limited slip. I have only had the truck a couple months, and checked the diff fluid over the weekend while I had it on a lift. The diff was way overfilled with what looked and smelled like ATF. Did the previous owner know something I don't, or should it be gear lube in there? The gears don't whine or grind.
Thanks

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so it was really runny? ATF is something like 20 or 30wt I'm guessing. gear is 90 and smells heavily of sulfur. yeah I'd change it in a heart beat. Even if there is any question in your mind.

it was really runny and red

ATF typically uses a 10-weight base oil, gear oil runs 75-90 at room temperature. I don't think I've had gear oil just "run out" after a period of rough service.

If you do change the gear oil, don't forget to get the LSD additive, or your clutches will start to chatter. Follow the instructions for mixing that you'll find on the bottoel - and it doesn't hurt anything if you use it in a regular axle, either. I've got one LSD in my fleet of four - but I just mix friction modifier in all my gear oil so I don't have to keep anything straight... And, even the back of the bottle sez it's not a problem if you use it in anything without clutches.

5-90

Don't think he needs that much FM... just get the little tube at your local parts store with the gear oil. Use a 75-90 or 80-90 or 85-90 GL5. squeeze the tube into the diff then top off with gear oil.

Nah - it's a little bottle. I think it works out that one little bottles does one gallon of gear oil - but I don't recall (I don't do it often enough to remember - I just restock fluids once a year for the shop.)

It's just with four 4.0 XJ's, I tend to maintain stock on a LOT of fluids, you see, and that may make my advice sound like it's "in bulk..."

5-90

Lots of good products out there, but my recommendation would be a product from Red Line called Shockproof. It can be used in conventional, limited-slip, or locker differentials. Another benefit of the Shockproof lubricants is that these gear oils do not develop the distinct foul-smelling sulfur odor which is a characteristic of conventional gear oils. Plus you dont have to worry about adding any of the friction modifiers.

pottenger said:

Lots of good products out there, but my recommendation would be a product from Red Line called Shockproof. It can be used in conventional, limited-slip, or locker differentials. Another benefit of the Shockproof lubricants is that these gear oils do not develop the distinct foul-smelling sulfur odor which is a characteristic of conventional gear oils. Plus you dont have to worry about adding any of the friction modifiers.

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" I love the smell of gear oil in the morning" Kind of stays with you all day.
Wayne