2015 F150 owners manual rear diff fluid options

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I am finally getting ready to change my diff fluid in my f150.
I grabbed some supertech 75w-90 after taking a look at the sds it's a group 3 base stock blended by Warren and looks fairly equivalent to Mag 1 going by flashpoint which are identical and viscosity which is similar.

Since the book specs 75w-85 on the main capacity page. I wanted to take a look at the severe service page way in the back.
So I found this blurb that appears to allow 75w-140 in situations where someone is actually using the truck as a truck.

In my opinion it looks like you can run 75w-85 up to 75w-140 according to Ford. It looks like you can skip buying the expensive 75w-85 from motorcraft unless you operate in Artic conditions or want that .1 mpg increase.

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), change non-synthetic rear axle
fluid every 3,000 mi (4,800 km) 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.
 
Until (what seems like) the CAFE bug hit Ford they specified 75W-140 in the 9.75" axles for decades. I would not hesitate to run it if you use the truck for anything more than a grocery getter. I ran Amsoil severe gear in my 2010 FX4 with great results:

[Linked Image from iili.io]
 
In an application calling for 75W85, you're right in saying :
Quote
In my opinion it looks like you can run 75w-85 up to 75w-140 according to Ford. It looks like you can skip buying the expensive 75w-85 from motorcraft unless you operate in Artic conditions or want that .1 mpg increase.

There is no such thing as 'correct' viscosity grade in gear oil recommendations, or engine oil recommendation for that matter should one decides on a viscosity grade or two higher .
 
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My uneducated take on this issue: engineers come up with ISO 220, and it gets 'translated' into SAE and AGMA grades. Then marketers step in, and we know what happens next. A mess. I am with Mr Zeng on this one, thank you for your continuous enlightenment.
After I poured Delo ESI 85W-140 into one of my 8.8 Fords by accident, I keep doing this for my other axles and from other makes. Of course, watch your temperatures.
 
I changed all my vehicles to SuperTech syn 75w-140 over the winter with no loss of mpg. This coming from Mobil1 75w-90. Eliminated the wine in both Ford axles. Very happy with the change.
 
I use the heavier 140 weight in my Ford trucks as well. The M275 in my super duty specs 140 so I am covered there, the F150 gets 140 because it used to tow near its max.
 
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