A guy was telling me the other day that he has a new truck, and at the first oil change he put amsoil in his engine ,trans and rearend and rised his gasmileage 3 mpg.
Actually the 8.8 holds a tad over 2 quarts. If you buy 2 quarts of fluid and the 4oz friction modifier that ought to be just enough. I bought a 3rd quart just in case I spilled some. If I could find it I would use Mobil 1 75W-140 but, most places don't carry it so I have been using Motorcraft's 75W-140. Just to save you the sticker shock, it is $17 per quart. I would also get one of those cheapy hand pumps in the gear oil section at the local auto parts store as it is a bear to get the new fluid back in without one. Make sure you have a tube of Permatex gasket maker for the cover. Make sure you clean the cover and the housing really well before you put the Permatex on otherwise chances are you will get a leak. The cover bolts should be torqued to 32ft-lbs and the fill plug 22ft-lbs. Give the Permatex about an hour to set-up before re-filling the housing. It is a 13mm socket for the cover bolts and you can just use a socket extension for the fill plug.
Here are the fluid specs.
Ford 8.8-Inch Ring Gear & Ford 8.8-Inch IRS
High Performance Rear Axle Lubricant SAE 75W140
Specification WSL-M2C192-A