Peter
General opinion over at
http://www.bobistheoilguy.com where they post used oil analysis of engines, transmissions and diff fluids.
Change gearbox and diff fluid at 30k miles to get the ware metals from running the unit in, still there at the 60k miles flush and get a true result at 90k miles after the wear iron is out the unit.
So good time to change the fluid.
Could not find any Ford gearbox UOA's of that gearbox.
eddie