Going to be installing a limited slip in my 2011 this summer. Its an older model (read discontinued) but new-in-box Eaton Posi, yup posi not trutrac. Anyways, Eaton specifically recommends 80w90 mineral gear oil plus friction modifier for this unit and I believe Ford recommends 75w140 synthetic for our rear dif's. Any thoughts?

Based on some googling, I suspect OE-type synthetic may be fine, but I'm not sure. Seems it could be a chemical compatibility issue with the carbon fiber clutches, or it could be an issue with excessive FM in aftermarket synthetic gear oil. Either way, I guess I'll have to get on the horn with Eaton tech line to try to sort this out....

 

If you were worried about excessive FM in the synthetic, get your 75w140 from WalMart. Their synthetic doesn't have any, but most name brands do. Getting it without the FM allows you the flexibility to dose it as much or as little as needed.

I'd probably break it in on mineral, then switch to a 75w140 or 75w110. I don't think our ring gears are happy with a 90wt. There's some math out there that can calculate what weight oil you need, but as best I can remember our 9.75" ring gears walk the line with 110/140.

There is the 85w140 mineral oils too, but they are pretty thick in cold wa
Weather, so id shy away from those, depending on where you live.

Good luck!