Kay, so I've got a couple questions regarding the 9.75 rear diff.

Fort, since my pinion is turned up, more so than a stock 9.75, should I put more gear oil in? I was always taught to fill till it starts to pour out, but I'm more into doing by the book. I got about 2 3/4 quarts of Royal Purple 75/140 gear oil in and It didn't come close to the fill plug. Should I add more or let it sit with what I've put in it?

 

Sure. Won't hurt anything. If its too much the pressure will push the excess out the breather tube.

 

Ford says that the rear differential should hold 5.7 pints of fluid. For the rear differential they call for using SAE 75W-140 GL-5 synthetic gear lubricant and 4 oz of friction modifier (Ford part number C8AZ-19B546-A).

Some of the gear oil has the friction modifier already in it but I am not sure about the Royal Purple you are using. I would check the label or call the folks at Royal Purple if in doubt. It also sound like the 2.75 quarts you added are exactly what Ford says the capacity of the differential should be. They also say, however, that you should fill the differential until it is level with the filler hole.

Speaking from expensive experience, failure to add the correct fluid will result in the failure of your differential. It won't happen right away but you can expect a failure a couple of years down the road. Especially if you operate your truck in high work load situations.

 

that would be an option but RP is expensive stuff! It shouldn't be too long and I'll be getting a bigger cover, so I'll have to swap this fluid again anyways.

The RP does have a friction modifier in it, that I don't actually need because I have a Tru Trac in the rear. No clutch packs, no friction modifier needed.

 

Yes, fill it till it comes out the fill plug. That level helps lubricate the inner pinion bearing.
As for fluid, just use the cheapest synthetic 75/140 you can find. Even the Super Tech stuff from Walmart works fine.

 

well looks like I'll be adding more when I get back to town! Spent the day shooting up in the hills. It's pretty cool to watch a 308 go against a bowling ball!