Hi all,

AC Delco 10-4061 Dexron 75W-90 Gear Oil is what is recommended. Unable to locate. Any recommendation where I may be able to purchase this or what its used to replace it. Thanks

ANY 75-90 gear lube is fine. Synthetics are ok, but not needed. Be sure to add a bottle(or 2) of positrack additive first. There are SOME gear oils that have a posi additive already in it, so that is also an option. 

Thank you

Amazon has Delco #4061, and some other varieties, plus the LS additive.   There is one that is designated LS and states no additive needed, but I have no idea how that would perform in a C-3 posi rear axle. This shows the #4061 on the bottle.

The synthetics, Mobil 1, Lucas, Royal Purple, usually have the LS designation and supposedly already have the LS additive in them.  At least that is what the label I read said.

"Mobil 1 Syn Gear Lube LS 75W-90 combines wax-free synthesized hydrocarbon base oils and a specially designed extreme-pressure, limited-slip, sulfur-phosphorous additive system to help provide a significantly higher level of performance in rear axles and differentials versus conventional fluids."

You can go with an all in one gear oil if you want but some don't work as advertised. You will know when the diff started to hammer on corners. You can also go with what has worked for years. A quality additive such as the GM 4 oz bottle (use 2) and a quality gear oil. In cars used in temps from 30*f and up I use Lucas 85-140 gear oil. I change it out at 8-10k miles as part of maintenance and it has worked for decades. The posi's I build do not have the spring pack in them, the stock ones do. If you still have snowflake clutches, you can use synthetic- although I never do. If you have fiber clutches do not use synthetic. Actually, I wouldn't recommend the fibers either.

Boom.  Thanks Gary.  Nothing else needs to be said.  Sounds like I need to R&R my diff oil. It has 20k miles on it.

Well a lot of them have gear oil in them for say 50 years. I have taken apart some that the oil was low due to leaks and over heated and they looked like they were powder coated black inside. Took me hours to clean them.