Rear End Oil Question

[jeffa55]

Instlaling a new rear end and have a couple of questions:
1 - I want to use a non-synthetic, but what weight is best? Recommended brands?
2 - How much oil does a dry rear end take to fill??
3 - Where can you buy the posi additive... only at dealerships?

Thanks!

[Vin63]

This is usually a personal preference thing...but, I use Torco 85-140 (slip additive already included) conventional to break-in my new gears. Then I go to a synthetic 75-90. Most of your good rear gear lubes have the additive included. The quantity depends on your rear end...I use 3.5 quarts in my Ford 9-inch, but my rear housing is heavily modified.

[vincef]

I was told running full synthetic may not be good in the rearend. Any ideas why somone would think this??

The statement came from the guy rebuilding my GM 12 bolt rearend. He said definately not during break in and does not recommend it after break in???

[1quik69]

What rearend are you putting in the car? Different rear ends take different amounts of fluid. Definitely us conventional oil for break in. You can go to synthetic later if you so desire.

[JBimmolate]

Some posi manufacturers recommend using non synthetic fluids. They have found through their own in house tests that running synthetic fluids with their posi units can cause premature wear and thus earlier failure. I only recommend synthetics for drag racing but that's my personal preference. Also some additives do not mix well with the synthetic fluids.

Most standard gm diffs take about 2+ quarts to fill.

The better additives are from dealer GM & Ford IMO

Oh and yes all GL-5 gear lubes have some additive in it already it is not enough for some of the HP posi units made today. GL-6 Gear oils have a higher additive content.

Miles