XL-18 is a friction modifier, much like XL-3 for the differential. If you've ever heard the 8.8" without XL-3 in it, you'd understand XL-3's quieting/smoothing properties. The formulation was likely changed from XL-3 to work in the MT82. Heck, it may even be the same stuff in a different labeled bottle, just so customers wouldn't ask why they are using differential friction modifier in their transmission.
What color is XL-18? Does it look dark blueish/green?
Weird......On the Amsoil webpage, there are no product recommendations for the Mt-82 in the 11+ Mustang GT. They list products for the Auto trans
Amsoil removed all MT82 recommendations from their website soon as customers starting having trouble with the transmission. Knowing it's mechanical failure, Amsoil wanted no part of it.