I just had my 12 bolt rebuilt and had a new Eaton posi carrier installed with the new gears. I asked the guy who rebuilt it what gear oil to use and he recommended 80W-140 and a bottle of limited slip additive, so on the way home I got some. Then when I read a slip of paperwork that came with the new carrier it said to use 80W90 with a bottle of additive, so I went back to the store and got the only 80W90 they had, Pennzoil Gearplus GL-5.

This stuff says it's especially suited for limited slip differentials and is an excellent replacement fluid for rebuilt limited slip units. It says you don't need a limited slip additive nor is one recommended.

My question is- do you guys have any experience with this gear oil and do you think I should still add the additive? I'm thinking if I called Eaton they'd say use the additive and if I called Pennzoil they'd say you don't need it.

Thanks,
Brian

Here is more to confuse you, I bought a new Eaton Posi and it said NO synthetic oil, well I use Amsoil 75w-90 that has the additive built in and it works great. Eaton has outdated info. on its little card in the box warning against synthetics, so it doesnt have to test its gears in every product on the market, it was easier to just say 'dont use syn'. The additive wont hurt, in fact some gear set ups at times have required 2 bottles of the stuff from what folks have told us in here. Toss in the additive, it wont hurt it.

I have been down this same road. I changed my oil to RedLine because the Summit catalog said it was required for warantee (by the way I have an Eaton posi). Then the new catalog said mineral based oil only. The truck section of the same catalog says Redline Oil "recomended". I called and emailed Eaton about thirty times before I talked to somebody. They told me synthetic oil was too slippery for the posi. I have no idea what that means! Eaton said to use any mineral based GL5 80w90 with the GM posi additive. I posted that info here and got a response that Amsoil will cover any warantee problems because they are sure that their oil will not harm the Eaton units. (was that you click?) I have just put in Valvoline 75w90 mineral based with GM additive and all the chatter I had with the RedLine Oil is gone. I don't know why, but I don't care. To be fair I have not tried the Amsoil to see if it does anything different, but the Valvoline seems so much smoother that I don't want to mess with it.

Put the oil in as it is and drive it for a while. If you get clutch chatter when making sharp turns or when parking the car then put the additive in. If you don't need the additive then you are better off without it. The additive lets the clutches slip easier, which isn't what you want when you want both wheels to spin together.

I just put new axles in my 12 bolt with 3:73 and Eaton posi. I used the Valvoline and GM posi additive. All's well

here is the letter from Amsoil its a Word Document

http://www.brainerd.net/~knudsen/69RS/Eaton%20LSD.doc

Thanks guys, I think I'll add half the additive bottle and see how it goes. If it's quiet I'll just let it be.

Brian