what gear oil to use after break in?

If you are running the factory posi, then probably what the book calls for. Now with that said, I had the tech at Ford tell me even though the book calls for 80w140 synthetic to use the 80w90 petroleum with 6ozs of the additive. If using a Eaton unit they have carbon plates and specify to use 80w90 with 6ozs of the additive. DTS recommends the same without the 6ozs of additive.

So, this all confused me and with the stock posi and stock 3.55's I use the factory spec 80w140 synthetic. With the 4.30's I tried the least expensive way using the 80w90 petroleum with 6ozs of the additive and everything seemed fine. With the Eaton Posi and the 4.56's I run 80w90 petroleum without the additive and have no chatter or problems so far in about 1500 miles of use.

I'm sure this can be debated forever like which engine oil to use but if you have no warranty concerns I'd say use the lesser expensive Valvoline petroleum oil and the Ford Posi-additive at either the 80w90 or 80w140 whichever you prefer.

royal purple