Gear oil?

**yellowv** · 02-23-2007, 03:41 AM

Hey guys I am rebuilding the rear in my Formy. What is the best gear oil to use? Brand and weight? I am going to use sythetic. Also I was told an Auburn differential doesn't need posi additive. Is that true? Also how many quarts do I need. Please help I want to order the last few parts and the oil today. Thanks. Oh yeah the rear will be just like stock just all new parts . New auburn diff, Motive 3:42's, Ratech deluxe kit and a TA girdle. Please no get a 9 inch or 12 bolt. I cant afford it. This is costing me way less than half.

**ss~zoso~ss** · 02-23-2007, 06:50 AM

ask Sarge, i'm sure Royal Purple makes a diff lube, i like them

**Fastcar** · 02-24-2007, 06:58 AM

Mobil 1 also makes a synthetic gear lube (GM markets one also) which will work fine. You don't need the additive if you use the synthetics, but if it gives you peace of mind put it in - it won't hurt anything.

**yellowv** · 02-24-2007, 08:41 AM

Thanks I'm going to go with the Mobil 1 just like in the engine and tranny. I also ordered a bottle of additive just for the heck of it.

**YoMommasTA** · 02-24-2007, 01:38 PM

i have used mobil 1 in my ta,i wouldn't use it again.Go with amsoil or royal purple.mobil 1 is garbage,so is the factory synthetic.now if you like replacing carrier bearing stay with the m1, i would put the regular cheapo diff oil in before gms or m1 garbage.its that bad.

**Fastcar** · 02-24-2007, 07:39 PM

Originally Posted by YoMommasTA

i have used mobil 1 in my ta,i wouldn't use it again.Go with amsoil or royal purple.mobil 1 is garbage,so is the factory synthetic.now if you like replacing carrier bearing stay with the m1, i would put the regular cheapo diff oil in before gms or m1 garbage.its that bad.

Sure it wasn't an existing mechanical problem you had before you used M1 or the factory synthetic? When my pinion seal went @65K my Pontiac mechanic replaced the fluid with GM synthetic. I have 93K on the car now with no diff. problems, but then again I don't beat on my car. Nothing in the diff. was touched except for the pinion seal.

**YoMommasTA** · 02-24-2007, 07:52 PM

well if you ever need to tear down again like i did you will probably find worn carrier bearings.Everything was setup ok,ran about 10k with m1,went to install a different carrier and wow were we shocked at the wear patterns on the carrier bearings.They weren't overheated or scored,they were actually chopped???strangest dam thing i ever seen..I went through 3 sets of carrier bearings using the gm synthetic and the m1.Another thing is those oils are super thin when compared to any other oils.Since i started using amsoil i havn't had any more trouble.I would use a regular non synthetic oil before m1 or gm's stuff.

**lrader99** · 02-26-2007, 09:49 PM

just did a new rear setup using an auburn so i called auburn and asked to talk to an engineer...they transferred me...i told him my setup...auburn richmond 4.10 moser 28's...i used to run only Mobil 1 synthetic 75w90 with 7 oz limited slip...he said not to use that with the auburn the reason is its a completely different type of posi...the auburn relies on friction to actuate, using a synthetic lube is too slick for the design characteristics of the rear and will actuall cause you to lose traction...he told me to use a good 80w90 and use 4 oz of limited slip...so i did...works great i used a havoline...i change my fluid quite often also like every 5k

**Fastcar** · 02-28-2007, 10:20 PM

Originally Posted by lrader99

just did a new rear setup using an auburn so i called auburn and asked to talk to an engineer...they transferred me...i told him my setup...auburn richmond 4.10 moser 28's...i used to run only Mobil 1 synthetic 75w90 with 7 oz limited slip...he said not to use that with the auburn the reason is its a completely different type of posi...the auburn relies on friction to actuate, using a synthetic lube is too slick for the design characteristics of the rear and will actuall cause you to lose traction...he told me to use a good 80w90 and use 4 oz of limited slip...so i did...works great i used a havoline...i change my fluid quite often also like every 5k

That's like the limited slip rear on my AMX (very old school) which actually has clutches in the differential: I use dino gear oil in that, Castrol with a friction modifier additive.

**INMY01TA** · 03-03-2007, 11:31 PM

Your not supposed to use synthetic in the Auburn.

**LS2Tuner** · 03-04-2007, 07:06 PM

We use the RedLine 85-145 out here in the desert heat.

**Wikid NaSSty** · 03-06-2007, 08:03 AM

Be very careful when it comes to when to use synthetics and additives. I have heard some bad things. I believe the F Bodies either came with an Auburn or a Torsen type diff. They have different requirements for lubrication and additives in order to make them function properly. I have a Zexel Torsen in my 00 SS and I am inspecting the rear and replacing the fluid next week. Anybody do any solid research lately as to which diff requires what types of lubes and additives?