For those of us towing, the manual is clear as mud or at least Mr B's fluid:
"If you are towing a trailer frequently in hot weather, hilly conditions,
at the gross combined weight rating (or any combination of these
factors), consider refilling your rear axle with synthetic gear lubricant
(if the axle is not already filled with it). See the Capacities and
Specifications chapter for the lubricant specification."
and
"Normal vehicle axle maintenance: Rear axles and power take-off units
with synthetic fluid and light-duty trucks equipped with Ford-design axles
are lubricated for life; do not check or change fluid unless a leak is
suspected, service is required or the assembly has been submerged in water.
During long periods of trailer towing with outside temperatures above 70°F
(21°C) and at wide-open throttle for long periods above 45 mph (72 km/h),
non-synthetic rear axle fluids should be changed every 3000 miles
(4800 kilometers) or three months, whichever comes first. This interval can
be waived if the axle is filled with 75W140 synthetic gear fluid meeting Ford
specification WSL-M2C192-A, part number F1TZ-19580-B or equivalent. Add
friction modifier XL-3 (EST-M2C118-A) or equivalent for complete refill of
Traction-Lok rear axles (see Technical specifications in the Capacities and
Specifications chapter for details)."