Differential fluid for posi [Archive] - Chevelle Tech

: Differential fluid for posi

Jun 12th, 03, 12:56 AM
What type and where can I get the proper differential fluid for a 12 bolt posi rear end for my 71 chevelle? My tech manual says use special positive track fluid but no one seams to carry it.

big gear head
Jun 12th, 03, 7:06 PM
I use Amsoil 75W90 Series 2000 synthetic oil in the posi rear ends that I build. If you want to use regular 80W90 gear oil you should use GM additive #1052358 with it. Get it at any GM dealer.

Jun 12th, 03, 11:16 PM
Gear Head,
I was reading the back of the Valvoline synthetic 75W90 and it said that an additive is not needed nor is it recommended is that true? What do you think of the redline "shockproof" line they sell at Summit and Jegs that say they are specially formulated to help rear gears that are exposed to 'shock' loading as in drag racing or romping? I also would like to fill my rear up now that I about killed myself installing the new gears and the infamous "crush" sleeve.

Jun 13th, 03, 1:50 AM
Yes, you can use the standard 80w90 gear oil with the GM additive, or use Red-Line for posi-rears and not use an additive. Summit or Jeg's has the Red-Line products.

Jun 13th, 03, 12:36 PM
would you still use the posi additive for the synthetic?

Jun 13th, 03, 2:28 PM

I don't know what kind of Posi you are running but listen to what I went through.

I'm running an Auburn Gear Pro series Posi unit. I tried all kinds of lubricants including Redline synthetic and still got a lot of noise out of the clutches when making slow turns.

I called Auburn and they told me to go to a GM dealer and buy their 85W90 gear lube and their additive. I put it in and the noise is GONE! I will never run anything else again.

I realize that the Auburn uses the cone clutch design and is different from an Eaton but I gotta believe that it would work well in either type unit.

JM .02


Jun 13th, 03, 3:27 PM
Originally posted by gja396:
What type and where can I get the proper differential fluid for a 12 bolt posi rear end for my 71 chevelle? My tech manual says use special positive track fluid but no one seams to carry it. Our local driveshaft shop also does a lot of rear-end work, and they recommend against any synthetic lube with posi units. They claim they see a higher rate of unusual wear with the rears running synthetics.

I trust their judgement/opinion enough to heed it, although I can't possibly verify if that's the real reason.

Jun 13th, 03, 3:31 PM
I run Redline with my 400# Eaton posi. The posi groans a little when you make tight coasting turns (such as into a parking space), but it doesn't chatter, so I don't worry about it. If I give it a little throttle to load it while turning the noise goes away. I had the same noise with Torco fluid with the GM additive. I've tried adding the GM additive to the Redline and it will still groan on tight coasting turns.

I prefer synthetic gear oil. I noticed a substantial drop in the temp of the rear cover (30F!) when I changed from the regular break-in gear oil to Redline synthetic.

The vendor who sold me my Eaton said the 400# units do make some noise in tight turns, it's just a side effect of the stiffer springs. A little noise from the posi in tight turns is OK, but chatter is not. That's hard on the clutches, spider gears, and your axles. If the noise suddenly went away on me I'd be worried the springs had failed or got weak.

I also run Redline with no additive in the rear end of my '89 Turbo Trans Am (Auburn cone posi) and it doesn't make a peep.

Redline, Mobil-1, AMSoil, and Torco are all good makers of synthetic rear end fluid that come to mind. Most say the additive isn't needed, but adding some certainly won't hurt things.

Mike, your comment of the rear end shop noticing unusual wear with posis and synthetics is intriguing (and worrying) at the same time. Can they give any examples characterizing the type of wear? I assume this wear is only on the posi unit's clutches/cones?


big gear head
Jun 13th, 03, 9:27 PM
First of all let me say that GM, Ford and Chrysler are all using synthetic oil in all new truck and rear wheel drive car rear ends. A good synthetic oil is much better that petroleum oil any day. As for the high shock loads, the film strength of the oil determines the shock that it can withstand. I don't know what the film strength of the Red Line oil is, but I know that the film strength of Amsoil is among the highest of any oil. The Amsoil 75W90 Series 2000 is the only oil I have used that didn't require the GM additive. Yea, I know I talk too much about Amsoil, so I'll stop. As for the shop that sees more wear on gears that have synthetic oil, something is wrong with this. Also, every Auburn posi that I have installed has required 2 bottles of GM additive to get them to stop popping. The Eaton posi is a much smoother working differential. Another thing to consider, the more additive you put in, the easier the clutches will slip, and the harder it will be for the clutches to lock up when you want them to.

Jun 13th, 03, 10:47 PM
MY '65's 12 bolt w/ Eaton started popping when cornering at about 90,000 miles. I replaced the OEM lube with Valvoline 90W GL-5 spec oil, worked fine ever since. I believe that any GL-5 lube is fine for limited-slip differentials. Valvoline, Shell, Chevron, etc. describe their products on their web sites. Google. Some very good "in general" info on the Shell aero lubricants website.

My $0.02