friction modifier

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Decided to change the rear diff oil in my F250. I have the Sterling 10.5 rear end which calls for 75W140 and friction modifier. The owners manual calls for 8 ounces of Ford-C8AZ-19B546-A, which is now the XL-3 friction modifier, for those with Traction-Lok axles, such as my truck. My question is, should I put in all 8 ounces, or only try 4 ounces as some have suggested? I also called a reputable shop in Missouri, Custom Differential, and they recommended 8 ounces. Ill be using Amsoil Severe Gear 75W140, and some have said that no friction modifier is needed with the Amsoil. whats the best route to take?
 
I think when I put in Mobil-1 a few years ago, I left the extra additive out.

The additive doesn't add life to the diff clutches, it just stops annoying chatter, if you have it. Mine doesn't chatter. Often 4oz is plenty.
 
Just add all 8 oz. If you add 4 and then still have chatter, it is hard to remove 4 oz of fluid then add the FM. Even if you use a fluid with FM, the Ford LS is very picky.

Also, the XL-3 is some of the smelliest stuff ever, use gloves when you handle it and don't attempt to wash any rags you get it on, just throw them away.
 
I can attest to friction modifier smelling absolutely terrible. Just got a Motorcraft stock order and one of the bottles of XL-3 was leaking. I got maybe 2-3 drops on my fingers and went running out of parts, through the business office, through the showroom, and to the bathroom to wash my hands. The entire front of the dealer now smells of friction modifier. There was a dude in the bathroom washing his hands when I ran in and he looked at me with this absolutely horrific look on his face and said "you need to go tot he doctor if it smells that bad."

This was probably payback for years ago playing pranks with it by putting a few drops into the AC inlet of the service managers office or taping an open bottle under the desk of a service writer that was bugging me.

Back on topic, when I had a car that had it I would use just one bottle of the XL-3 when changing fluid on an 8.8" diff. Drive the vehicle in a figure 8 to work it all in and check for chatter.
 
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I recommend adding 1/4 tube at a time and then warming up the differential.

Do a figure 8 in a parking lot.

Only add enough until chatter has disappeared.

Too many people over-treat their diffy's with this additive.
 
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thanks for all the replies guys, they are all helpful. I'll be changing the gear oil tomorrow, I'm still hesitant on whether I should only put 4 or 8 oz. I've heard that if I put too much additive it will be a "limited grip " differential and wont work properly. I figured if the manual says to use 8 oz it cant hurt, but then again, who knows if it will be too much?
 
will the chatter be noticeable in the cab? It is a 7.3 Powerstroke, its pretty loud in the cab with the engine running, if it starts to chatter I hope I can hear it
 
Yes, you will feel it and probably hear it too. I had a'99 and changed the lube with Amsoil and didn't add the modifier. Within a couple of thousand miles it started chattering so I added the modifier and it quit.
 
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