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Per 2013 Owners manual page 298 listed in the note section. I know a few people have decided not to use it including myself on my 2012 Boss. But since Ford added this statement to the 2013 owner manual I have since decide it best to follow ford's guidance.
1. Add 4 oz. (118 ml) of Additive Friction Modifier XL-3 or equivalent meeting Ford specification
EST-M2C118-A for complete refill of Traction-Lok or TORSEN® axles.
Ford design rear axles contain a synthetic lubricant that does not require changing unless the
axle has been submerged in water.
When subjecting your car to high-speed or competition use, the axle fluid and friction modifier
should be changed after the initial (first) hour of high-speed operation or if the vehicle is
subjected to track or competition conditions; thereafter changing the axle lubricant and friction
modifier every 12 hours (under these conditions).
Most all rear lube will have a friction modifier in it. Ford's sh*t isn't special, so the modifier is in there as per the Ford recommendation. If you use Royal Purple Max Gear 75W-140 w/modifier you'll be fine. This is all I put back in, and no issues at all.
It's just going to make the oil stink. Seriously, why use it? It's purpose is to change the friction on clutch disks. The Torsen has no clutch disks. I was with Rick when we discussed this with Ford Racing.
I'm not going to buy that bottle of Ford friction mod., but I will purchase gear lube with the mod. in it. I know the torsen does not need the modifier, but it is written in the fluid maintenance specs. If my rear goes I'm not willing to give a Ford dealership any hope at attempting to blame it on me. It's not going to hurt anything so I use it, and it might get you fixed and back on the road sooner. Ford Racing knows what their taking about, but they don't perform warranty work.