I have an early 2012 Mustang GT Premium Brembo. I too am experiencing the notchy shifting between 1 and 2 and sometime 3rd gear, especially as it gets colder. I bought the car used last Feb with 17000 miles on it and now have 23000 miles on it and couldn't be happier with the purchase as my DD. I went to the dealer and told them about the TSB to change the original trans
fluid and the new Trans
Fluid Chart, all of which I learned about on here. They were not aware of the TSB or the additive but indicated I qualified for the TSB and they ordered the XL18 on the spot, after I showed them the new fluid change chart and scheduled me a date to bring my car in for the trans
oil change. I will update the forum as to how it turns out. Ive added the GT 500 axel backs, the Barton shift knob extension and old school shifter ball, and Hawk Ceramic Street brake pads and new rotors as a result of what I have learned here and they have all improved my Mustang experience.