on new cars i like to do it at the first oil change, to get any metal shavings out of there. ford says that it is not needed unless the axle is submerged under water or service is done to the axle assembly, such as a bearing replacement.
depending on the year of the vehicle you use either 80w90 or 75w140. there will be a tag on the cover that says "use synthetic oil only" if 75w140 is required. basically any name brand gear oil will do, there not much difference really. just make sure you add some friction modifier unless you either have a non clutch type lsd or the fluid you use already has it mixed in.
What fluid do you guys reccomend - I'm going to take my car in for the 3.73's next week and I'm proiding my own gears and bearing kit. What brand of fluid do you guys reccomend? It's a 2001 Automatic.