Feathermerchant

From 2011 owners manual P275

If you will be towing a trailer frequently in hot weather, hilly
conditions, at GCWR, or any combination of these factors, consider
refilling your rear axle with synthetic gear lube if not already so
equipped. Refer to the Maintenance and Specifications chapter for
the lubricant specification. Remember that regardless of the rear axle
lube used, do not tow a trailer for the first 1,000 miles (1,600 km) of a
new vehicle, and that the first 500 miles (800 km) of towing be done
at no faster than 70 mph (113 km/h) with no full throttle starts.

smokeywren

The Owner's Guide for my 2012 F-150 specifies SAE 75w140 Synthetic Rear Axle Lubricant. I'll bet yours does too.


Way back when, the Owner's Guide for my 1999.5 F-250 diesel specified SAE 75w140 Synthetic Rear Axle Lubricant.



Your rear differential gets hotter than a two-dollar pistol when towing heavy trailers in hills and mountains. So it requires synthetic lube so the heat won't kill the lube.


My Owner's Guide also says that if I insist on being a hard-head and run dino rear axle lube, I have to change it every 3,000 miles. But if I comply with Ford requirements to run synthetic, I never have to change the rear diff lube.

Feathermerchant

I did find the fluid spec calling for synthetic further into the manual. That makes it even stranger. Why wouldn't it just come with synthetic from the factory?

On my 1995 Ram I noticed a hot differential at 75,000 mi and replaced the oil with Amsoil synthetic and never had a hot differential again.

smokeywren

Your F-150 came from the factory with synthetic rear axle lube. Are you asking about a different truck?