would like to start using a synthetic oil ( royal purple ) but my diffrential requires 80w-90 from the factory , id like to use RP but only offer 75w90 would it be safe to go to a lighter weight i have the BorgWarner diffrential on my 85 t/a

 

I have been an Amsoil dealer for about 11 years. I used Amsoil in most of the rear ends that I have built, which is probably over 400. I never had a problem with Amsoil synthetic oil. I cant say how RP will work. Going from 80W90 to 75W90 isn't going to hurt anything.

 

I run Lucas 75w90 in my diff.

If you have a posi that requires an additive, you still need to use the additive.

You can change all the oils in your car to synthetic if you can afford it.

 

The additive is more for the clutch style posi units. BW rears are cone so he can just run the straight gear oil with no additive. Im running mine without and it works just fine. I did a ton of searching on this board through past posts about 9 bolt gear oil before i was comfortable doing this however. It went against everything i thought i knew.

 

Just change it fairly often. The cone type posi puts a lot of metal particles in the oil as it wears. No matter if it's organic or synthetic, just keep it clean.

 

That's why I said "If you have a posi that requires an additive"

 

Clutch plates and cones usually require some of the additive. The Eaton doesn't need more than one bottle. An Auburn usually need 2 bottles. The Borg Warner may only need one bottle. If you can drive it on the street and it doesn't pop or chatter when turning then that is fine. Don't use more of the additive than you need. The additive causes the differential to slip easier, so if you are drag racing and doing burnouts then you don't want it to slip. Too much additive can cause problems.

 

i do still have the tag that says "limited slip oil only" i was just woried from converting if i would have problems cuz it has over 100k on it

 

Yea from my experience the BW doesnt need it. I drove mine fine and it didnt pop or make any weird noises on the street. And when i reshimmed it i shimmed it TIGHT.