Your car may have had it added at the factory, maybe.
Have you ever had a transmission fluid change?
I did mine at 1200 miles after new.
Transmissions have a break in period and sometimes small metal flakes are present in the oil.
Rather than potentially doubling up on the friction modifier, I think it would be best to go have the fluid drained, refill with new QDC and add the XL-18.
That's what I would do.
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.