Follow the manual.

I do run 75w-140 in my 18 raptor. Only because the engineer recommended it for Baja.

This may also help.

 

Mine came with 75w85. I changed it to 75w140 after 20 miles or so. Same axle as ford has used for years with a 75w140 spec magically needs a lighter oil all of a sudden.

 

Do you think the lighter oil will magically harm it ?

 

Probably Ford trying to get that last .5mpg on the rollers 😒

 

I am only assuming here…but the lighter oil would reduce drag in the rear end helping to improve fuel mileage which is a major selling point of the Ecoboost engined trucks. Heavier oils while providing more shock protection would increase drag and reduce fuel mileage.
I have to believe that any mileage decrease would be negligible unless you’re trying to hypermile your truck. I would opt for the thicker fluid if you were going to any serious drivetrain shock inducing maneuvers.

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Isn't this another instance where Ford changed their "recommendations" for fluid during the ownership of the product?

In other words, on the day you acquired the vehicle, Ford recommended a specific viscosity, and then later changed the recommendation for the viscosity?

I'm only pointing out that this does happen, and if nothing else it undermines the theory that Fords recommendations are unequivocally the best choice no matter what. Afterall, they actually change their mind at times.

There's a REASON Ford (or any manufacturer) would do that.

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Thanks all!
Ordered the 75/85 from A. Dealer wanted 60.00 a qt. A is 37.00 a qt. Fill plug was about 1/4" thick with filings. Hope that's the worst of it.
Motorcraft OEM oil.

 

Probly all break-in.

 

I put 75/140 in mine because I already had enough in my garage for a change. Haven't noticed any difference. I'd use it again.

Just use 75/90 if you want to stay close to factory rec but have something more readily available.

 

Since this is a rear diff thread I have a side question. I put the Ford performance LSD for the super 8.8 in my 2017 and I keep getting audible noise and I can feel the clutches slipping when turning but it's often not smooth slipping, more of a bind/slip/bind/slip kinda deal.
I put amsoil severe gear 75w-90 with the amslip sliplock friction modifier about 15000 miles ago. Should I be using something else or is this normal operation for an LSD?
I've never used a clutch type LSD before so I have no personal comparison.

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I put the 75w140 in because I regularly tow 7500 lbs with the truck loaded to the max door sticker weight. I also had 4 bottles of 75w140 left over from my 2009 F150. Same 9.75 axle.
Add another bottle of friction modifier if your differential clutches are chattering. If that doesn't help, you may have an actual problem. Ford XL-3 friction modifier is an option.

 

I put the 75w140 in because I regularly tow 7500 lbs with the truck loaded to the max door sticker weight. I also had 4 bottles of 75w140 left over from my 2009 F150. Same 9.75 axle.
Add another bottle of friction modifier if your differential clutches are chattering. If that doesn't help, you may have an actual problem. Ford XL-3 friction modifier is an option.

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Are you using the same LSD just on the 9.75 axel?

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I use 75w90. For those looking for a happy medium, 75w110 are quite available these days.