I got a decent mechanic. I am coming up on 100K for the 2007 GT. The manual says to use the proper weight fluid for the diff plus friction modifier, I believe 4 oz. Mechanic says I don't need the modifier. What say 'ye?
Why wouldn't you need it? More importantly, why wouldn't you just spend a couple bucks to be safe?
Some synthetic fluids claim you don't need it, but you know what? Those guys don't have the information about your vehicle that Ford has; they're guessing.
Look for a black square with the white letters "LS" on the label. If it's not there, you need to add FM separately. Motorcraft diff lubes needs the separate FM, but Valvoline and Mobil 1 do not...I'm sure there are others as well.
Your mechanic is correct. The majority of brand name gear oil suppliers contain an LSD additive from the factory, no need to add a supplement. I have always used QS and still do.
The Ford fluid does not come with modifier already mixed in so it must be added. Your Mustang does have a clutch type limited slip, so you need to make sure that there is a modifier in there.
I still remember the day when I learned to rebuild a diff. It was a '98 ranger with a LSD. When it was time to refill the rear end, I didn't have any 75w-140 which was specd for it so I filled it exclusively with Lucas HDOS. It worked without a hiccup for years.
I use royal purple in my differential and even though they say it has a friction modifier inside I still added a bottle of Mopar friction modifier to it. A whole $7 and no risk of chirping around or breaking any metal components inside. Just add it. Better safe then sorry.