It's very unlikely for the differential to have both a locking mechanism and clutch packs for limited slip operation. Simply not enough room in the carrier. Nevertheless Valvoline SynPower 75-140 is, in my opinion, the very best you can get for a Ford axle. It has perfectly sufficient and effective amounts of friction modifier included, and if it has a mechanical locker, this doesn't even matter.
Ford originally spec'd 80-90 conventional for the diff in my '03 Marauder, and later published a TSB due to "soft axles" failing where the rear axle bearings ride, due to poor heat treating. Mine ate it's axles up at 105k, I put new aftermarket axles and bearings in it, and after a thorough cleaning and inspection replaced the fluid with SynPower 75-140, obeying the TSB. My 3.73 Traction-Lok has been quieter, chatter free and performs flawlessly ever since. I drag race the car a few times a year and beat the snot out of it when I drive it.
In conclusion, I highly recommend SynPower in any Ford axle. This car uses Valvoline Synthetic products exclusively. Not a Valvoline fanboy, just happy with my results. I use Mobil 1 products exclusively in my wife's cars, including gear lube. Her AWD Hyundai Santa Fe has 178k trouble free miles with Mobil 1 in the rear diff and X-fer case. Only because she is a Mobil 1 fan, and stubborn. She's a gear head too. I think her affinity for Mobil 1 comes from being a Mopar girl, and SRT cars are Mobil 1 factory filled. (She's had a couple of Challengers...)