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I'd use the 75w140 in a synthetic which should include the modifier for the limited slip. It should also be rated GL4 or higher.

Then after driving for a while and if you notice a chirping coming from the rear end when making a tight turn consider putting in some of the modifier fluid

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Ok I appreciate the reply ! I called another dealer and they said it's in the manual. Ok so I have owned the truck from day one 200,000 miles on it. So the manual does say 75 90 for the dana 80 which is what I have but if the 75 140 will work then that's what I will put in.
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Ford will change their oil specs from time to time so that is quite likely where the 75w140 comes from.

But as I mentioned check the oil ratings on the bottle and see if it is GL4 or higher. If it is lower you will likely need to add a modifier. Most synthetic oils will be GL4

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Well I used the 75w140 but I got to thinking after a lot of reading and I wondered if they call for the 75w90 in the diffs where the fluid is also going to the axles and bearings where as the other diffs the bearings are packed with grease. It's just a thought.....
 

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Well I used the 75w140 but I got to thinking after a lot of reading and I wondered if they call for the 75w90 in the diffs where the fluid is also going to the axles and bearings where as the other diffs the bearings are packed with grease. It's just a thought.....
The SRW 10.5" rear wheel bearings are also lubricated by axle lubricant and specify the 75w140, so that shouldn't be an issue.
 
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