Technical Gear Oil Eaton Posi

I am in the process of building a 12 bolt rear with an Eaton Posi (clutch type). This one to be exact
https://www.summitracing.com/parts/etn-19556-010
Lucky for me I have been hanging on to it for a long time. Summit recommends a synthetic 80w-90 and an additive. Eaton says non-synthetic and no additive. The internet says 85w-90 from amsoil (synthetic) AND 80w-90 from Lucas. So what do you Eaton owners use for your posi diffs?

…Yes this question was answered 10 years ago but back then Victoria Secret sent a catalog and the models were tall and thin : )

 

I have a similar Yukon unit and it wants non synthetic and friction modifier. I was researching the Eaton unit for a friend and I’ve read NO synthetic with clutch style units.
Personally I would go with what the manufacturer recommends but I’ve always seen a modifier required. Just my 2 cents. I’m interested in seeing what others have to say.

 

Tryed synthetic in a 64 Chevelle 10 bolt and worked fine with the 225-75-15 street tires, but with 8" slicks it changed to a one wheel peel on the sticky drag strip.
Changed back to the conventional oil with a posi additive and it's much better. "it might be time to rebuild it"

 

Penn Grade GL-5 oil is what you want to use. We have tried many others over the years and have had issues with them all. We went exclusively to Penn Grade 6+ years ago at our Hot Rod Shop and it works flawless. Everything is in the oil, do not add anything else to it.
PennGrade 1 Limited Slip GL-5 Gear Oil 77326

You can get it at Summit Racing: PennGrade 77326 PennGrade 1 Limited Slip GL-5 Gear Oil | Summit Racing

 

Whatever brand and weight you decide make sure it is GL-5.

 

Thanks for the input all! I called up Eaton and they told me mobile 1 synthetic

I bought two bottles of the penngrade 1. If it hasn’t caused any issues for AHotRod in a Hot Rod shop for so long, it’s got be good.

 

I've always used straight 85w or 90w and always a friction modifier.