so i just picked up a borgwarner 9 bolt 3.27 posi rear end out of a 89 transam, cleaned it all up and painted it, but now i need to know what type of fluid everyone reccomends, and how much to put in..... i've never touched a posi rear before, and read some where to use a synthetic with gm posi addative, what fluid should i use, what ratio of synthetic to posi addative, how much do i put in, if you cant tell i have no idea on what to do, so a little help would be great

80w90 or 75w90 synthetic gear oil

Put in GM posi additive available from your GM dealer.

Yes, use the synthetic gear lube. You'll need about 2 litres.

The posi addative can be picked up from your local GM dealer. It's a small plastic bottle about 40 mls and sells for $5 or $6 bucks. Add the entire bottle with the gear lube.

most of the new sythetics gear lubes out today will tell u if they have the posi additive already added, i would get the best. i would put in lucas!!!! that **** is amazing u cant beat that....100% pure petroleum! im sure they make a posi lubricant if not u can always just use it and add the posi additive. i would check with lucas tho before im really not sure.

i always thought synthetic in a rear is a no no

thanks everyone i finally have an idea now, another question, whats the plug on the back of the differential cover for, just for filling with fluid? or is it like a sight also, so you now how much oil you have in there/, should it be filled up to the bottom of the plug area or is that too much? i know "freestylzz" said to use about 2 liters plus the posi addative, would 1 be not enought, where things could get damaged? would 3 be too much? just wondering

like i said earlier i know nothing about differentials, so i want to learn as much as possible to prevent me doing something stuppid and ruining it,

thanks again everyone

Yes, that plugs the filler hole.

1. drain the differential by removing the cover
2. check for and clean out any metal shavings, sludge, etc., in the bottom of the differential
3. clean off any old gasket material and sealant from the cover and differential mating surfaces
4. before replacing the cover, pop-out that plug
5. replace the cover using a new gasket and sealant, following the sealant's directions regarding its set-up time(some people might use sealant only, and some might use a gasket only... you'll prolly get a different responses for this)
6. pour in the bottle of additive first
7. pour/pump in the gear oil until it begins to run out that hole
8. that hole is also the fill level, so let it drain from there until it stops
9. pop the plug back in
10. be sure it's not leaking around the bottom of the cover, which, hopefully, you'd have noticed before you'd gotten this far
11. clean everything up
12. you're ready to go.

Too much detail makes it seem complicated, but it's fast and easy. No worries.

Lafireboyd, thanks.... thats exactly what i need to hear, so i'm pretty much ready i'm going to pik up the oil/addative this weekend and some silicone and i should be ready to go thanks again _ scott_

I hate to throw another wrench in your decision of which fluid to use, but I remember reading a post on here a couple of years ago from a guy who put the synthetic fluid in a nine bolt posi rear, and it started chattering around corners, even with the additive. He switched back to regular 80/90 oil with the additive, and it fixed the problem. I remember someone saying the synthetic fluid doesn't agree very well with the cone style clutches in the nine bolt posi units. Of course this could've been just one guy's experience. If anyone else on here is running the synthetic successfully in a nine bolt, feel free to chime in.

argh... u just had to rain on my pariad "joke" now i really want to know what everybody with a 9 bolt posi rear is using in the diff. i searched before i maid this post a week ago and i saw some post that said not to use synthetic, but most said to use it, so all you 9 bolters let me know what ur running in ur rear

Yeah, sorry about that. I hate to add more indecision to your choice. Believe me, I know how that goes. Lol. I did notice in this post that a couple of the guys who said to go ahead and run synthetic have a nine bolt rear listed in their info margin, so it's probably one of those 50/50 type questions. You'll more than likely be ok with either. Personally, I don't run synthetic anyways since I really don't see that it's a huge advantage when it comes to rear axles, especially considering the extra cost for it. I've run regular 80/90 gear oil in all my rear axles since I first started messing with cars almost 20 years ago, and I've never experienced a bearing failure or excessive wear in any of them. Just make sure to keep it up to the fill line and change it every so often. Of course I always make sure to put the limited slip additive in them too if it's a posi rear. Full synthetic or synthetic blend type motor oils definitely have their advantages over standard dino stuff when it comes to engines, but I just can't see the big deal in running it in a rear axle.

why dont you ask these guys

http://www.bwauto.com/contacts/

http://www.thirdgen.org/techboard/tr...dline+gear+oil (Rear End Gear Additive, Part 2 (9-bolt owners please read!))

I was told by a few shops that rebuild '9 bolts' all the time to use Redline 75W140 with NO additive. The cones will bang and clunk a little bit, but this will be totally normal and doesn't hurt anything. If your cones are worn, use the Redline heavy shock oil, otherwise the normal stuff will do. The additve is a friction modifyer or reducer and reduces the effectiveness of the posi unit (cone type).

** Edit oil was Redline 75W140 #50304

9-bolts don't need additives. Even my Factory Shop Manual calls for straight gear oil, no additive, for the 9-bolt.