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Differential question

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98
67
Exp. Type
HPDE
Exp. Level
Under 3 Years
Cantley
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?
Thanks!
 
6,676
8,784
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.
 

Dave_W

Cones - not just for ice cream
1,062
1,388
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
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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