Differential Fluid
Thread Starter
Junior Member
Joined: Dec 2024
Posts: 21
Likes: 9
From: NY
Differential Fluid
I have a 2021 F150 XLT 4WD with the 5.0. My transmission was just rebuilt and I doubt the differential fluid or transfer case fluid has been changed (I am the second owner). Looking to replace the differential fluid (front and rear) and I checked my manual for the proper amount. It gives two options, depending on the type of axle. One includes a friction modifier. How can I determine which axle I have and whether I have to add friction modifier?
Senior Member
Joined: Dec 2020
Posts: 859
Likes: 470
From: Pittsburgh, PA
To my knowledge, there is no clutch-based limited slip option in the 2021+ series. There is an optional Torsen differential for the front axle, but that is a helical gear limited slip, not a clutch-based one. This is only offered as a factory option on Tremor and Raptor models (that I recall). Otherwise it is open. For the rear, the options are open or locker.
So there should be no factory differential that requires limited-slip additive.
As for oil weight, I generally use 75W140 in all my differentials. I do have the max tow axle (9.75 3/4-float) in the rear, and since my front axle has the EIWEs, it is almost never spinning. (I also use it in my TJs Dana 44s.) If you have the Super 8.8 or standard 9.75 rear, you can also use 75W85 or 75W90.
If you have a significantly larger tire size than factory, it is better to err on the side of higher viscosity, as larger tires mean slower ring and pinion rotation and higher average and peak contact force. So with larger tires, you actually get less energy loss from the same fluid because of the lower gear speed, but also less of a protective effect (fluid squeezes out faster and cycles through less frequently). Going to a heavier weight fluid counteracts the squeeze-out effect at the cost of a bit more energy loss.
So there should be no factory differential that requires limited-slip additive.
As for oil weight, I generally use 75W140 in all my differentials. I do have the max tow axle (9.75 3/4-float) in the rear, and since my front axle has the EIWEs, it is almost never spinning. (I also use it in my TJs Dana 44s.) If you have the Super 8.8 or standard 9.75 rear, you can also use 75W85 or 75W90.
If you have a significantly larger tire size than factory, it is better to err on the side of higher viscosity, as larger tires mean slower ring and pinion rotation and higher average and peak contact force. So with larger tires, you actually get less energy loss from the same fluid because of the lower gear speed, but also less of a protective effect (fluid squeezes out faster and cycles through less frequently). Going to a heavier weight fluid counteracts the squeeze-out effect at the cost of a bit more energy loss.
Senior Member
Joined: Apr 2025
Posts: 364
Likes: 196
I have a 2021 F150 XLT 4WD with the 5.0. My transmission was just rebuilt and I doubt the differential fluid or transfer case fluid has been changed (I am the second owner). Looking to replace the differential fluid (front and rear) and I checked my manual for the proper amount. It gives two options, depending on the type of axle. One includes a friction modifier. How can I determine which axle I have and whether I have to add friction modifier?
Thread Starter
Junior Member
Joined: Dec 2024
Posts: 21
Likes: 9
From: NY
To my knowledge, there is no clutch-based limited slip option in the 2021+ series. There is an optional Torsen differential for the front axle, but that is a helical gear limited slip, not a clutch-based one. This is only offered as a factory option on Tremor and Raptor models (that I recall). Otherwise it is open. For the rear, the options are open or locker.
So there should be no factory differential that requires limited-slip additive.
As for oil weight, I generally use 75W140 in all my differentials. I do have the max tow axle (9.75 3/4-float) in the rear, and since my front axle has the EIWEs, it is almost never spinning. (I also use it in my TJs Dana 44s.) If you have the Super 8.8 or standard 9.75 rear, you can also use 75W85 or 75W90.
If you have a significantly larger tire size than factory, it is better to err on the side of higher viscosity, as larger tires mean slower ring and pinion rotation and higher average and peak contact force. So with larger tires, you actually get less energy loss from the same fluid because of the lower gear speed, but also less of a protective effect (fluid squeezes out faster and cycles through less frequently). Going to a heavier weight fluid counteracts the squeeze-out effect at the cost of a bit more energy loss.
So there should be no factory differential that requires limited-slip additive.
As for oil weight, I generally use 75W140 in all my differentials. I do have the max tow axle (9.75 3/4-float) in the rear, and since my front axle has the EIWEs, it is almost never spinning. (I also use it in my TJs Dana 44s.) If you have the Super 8.8 or standard 9.75 rear, you can also use 75W85 or 75W90.
If you have a significantly larger tire size than factory, it is better to err on the side of higher viscosity, as larger tires mean slower ring and pinion rotation and higher average and peak contact force. So with larger tires, you actually get less energy loss from the same fluid because of the lower gear speed, but also less of a protective effect (fluid squeezes out faster and cycles through less frequently). Going to a heavier weight fluid counteracts the squeeze-out effect at the cost of a bit more energy loss.
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