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Differential question

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Exp. Type
Exp. Level
Under 3 Years
2024 GT PP Magneride manual.
-yesterday I changed the original differential oil at 4800 miles (this mileage included 3 hours of track time in 20-30 minute sessions)
-today I take the car out and did a few pulls from a stop. Flat surface, wheels straight, I notice the rear wants to go to the left a little bit.
-prior to this, I always launched straight on the original fill.
-I used Amsoil 75w140. It said on the manufacturer's description that the product contains a friction modifier so I did not add a bottle of Motorcraft XL-3.

Is it just a fluke that I happened to have several launches where the rear wanted to go left, something that happens every once in a while, and it just happened to me a few times in a row? Nothing else has changed on the car.

Or should I flush this and start over with 75w90?
The friction plates are saturated with whatever fluid you ran before. It might take awhile for the new stuff to take affect.
FWIW I ran some stuff that supposedly had the friction modifier in it, I still had to add the Ford stuff.
Trannys use a friction disc similar to an automatic that replaces the brass synchros in new transmissions. I had one that just refused to shift smoothly at an autocross. Come to find out, the prior owner ran some zoomy aftermarket lube. I changed back to the original fluid and it improved, but was never great. I finally pulled the trans and replaced the synchros and replaced the fluid, the problem was finally solved. After that experience, I only used the factory recommended lube in everything.


Cones - not just for ice cream
Exp. Type
Exp. Level
20+ Years
I think the Perf. Pkg. comes with the Torsen diff, so no clutch discs. Strangely, it looks like the owner's manual still calls for friction modifier on both the (helical gear-type) Torsen and (clutch-type) Trac-Lok diffs, though.

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