You could keep the QDC fluid type and add the X-18.
Or go with the BG Synchromesh II. Some really like this. But if going this route, change it often. I hear it begins to degrade after 10K.
But I am no expert.
Me? I fretted and worried and obsessed over what oil to switch to, even ordered some MTL that I have sitting in the garage.
In the end, I refilled with QDC with 1200 miles on the car.
2012 Shelby American Mustang GTS 580rwhp 485 ft/lbs Grabber Blue White Stripes
Ford Whipple, 5.0L, Tremec XL Magnum, MGW, Centerforce DYAD clutch, Boss 302 Torsen w/4:10s, 302R EPAS, Eibach Coil Overs, Fays 2 Watts Link, Dynotech DS, Shelby UCAs/LCAs, Wilwood brakes 6 piston 4 piston, Drake Bump Steer, heavy duty cooling, welded axle tubes, Interior Package, GT4 wheels, Borla Touring axlebacks, GT500 rear valence w/parking sensors, and airbag cover signed by Carroll Shelby.