Best Fluid for 8.5 rear differential

[rfalker1]

Just wondering whats the best fluid for my rear differential? I am replacing and putting a posi, and I want a great fluid with a long life, that cuts down on friction.

[big gear head]

What differential are you using? That is very important. If you are using a Eaton then a regular 80W90 with GM or Ford oil additive will be just fine. Only use GM or Ford additive. If you are using an Auburn then you may need 2 bottles of additive. If you are using a Truetrac or Detroit Locker then use 80W90 or 80W140 without the additive. I have used Amsoil synthetic oil for over 13 years with no problems in any of these differentials, but most manufacturers don't recommend synthetic oils. You don't want to cut down on friction too much or your limited slip differential will slip too much and allow one wheel to spin.

[rfalker1]

Hmmmm, so nothing synthetic back there, and the differential is something like an Eaton, its the Yukon Dura-Grip Posi Unit 28 Spline, so what would be your final recommendation for this

[big gear head]

The Duragrip is the same thing as the Eaton, except for the clutch material. Use the 80W90 with GM or Ford oil additive.

[rfalker1]

Thanks, I am getting some Valvoline non synthetic and a lucas oil additive, Will that be ok?

[Northeast Rod Run]

I literally just finished reading an article on gear oil 5 minutes ago in Camaro Performer, and they had a Red Line rep suggesting their 75W90 and said that only the tightest clutch diffs would benefit from adding any modifier because their stuff is good right out of the bottle

I've always used synthetic in my rearend fluid changes

[CRCRFT78]

So what about a bone stock 8.5 GN 10 bolt posi, what do you suggest?

[big gear head]

I used Amsoil 75W90 in these rear ends for many years with no problems at all. The only reason that I don't openly recommend using synthetic oil is that the manufacturers do not want you using synthetic oil. I don't know why because all of the new trucks come with synthetic oil in them and they have the very same clutch material in them as the Eaton Posi and Duragrip. I leave it up to you. If you don't feel right about using synthetic oil then use a good petroleum oil with GM or Ford additive. I don't recommend any other additive because I have seen many others that just don't work at all. I have not used the Lucas additive so I can't give an honest opinion on it.

This goes for the new Eaton and Duragrip, as well as the GN and other clutch plate type limited slip differentials. The cone type limited slip differentials may require more additive than the plate types.Do not use more additive than you need to smooth out the turns.

[rfalker1]

Ok so if I go with Amsoil synthetic oil, because I hear two recommendations that means that I don't need the additive, and about how many quarts do I need to buy? Or do I still need the additive, plus if so can u tell which additive to use (brand).
Thanks for all the help

[rfalker1]

Lol Sorry bonehead mistake... Only only recommendation, but a 13 year one ;)

[rfalker1]

I have did some research and alot of people recommend the redline 75w 90 oil also with no additive

[NOT A TA]

I run Amsoil, have for many years in several different cars and rear types including stock 8.5 GMs. I put 75-90 in most of them and the "Severe Gear" 75-140 in the rear I use for the road race tracks because of the extended use with high temps.

[big gear head]

If you use Amsoil you need to get the Severe Gear 75W90. Amsoil makes several gear oils, but the Severe Gear is the only one that usually doesn't need the additive. The only time I have had to use the additive with this oil was with the Auburn differentials.