I would fork out the cash for XL-18 additive and not mess around with the differential friction modifier stuff. I'm a bit cautious, lol. This transmission is a bit sensitive to fluid and I am not sure if friction modifier would do the same thing as the more expensive XL-18 stuff. The cost difference is small compared to the cost of a rebuild (in the worst case).
"Bob is the oil guy" (BITOG.com) may have some good info on XL-18 vs. friction modifier. I'll go see if I can something worth re-posting.
2012 GT 6MT - RTR #59; performance white / charcoal interior
3.73's, AED tune, MGW Race Spec shifter, Airaid MFT, BMR rear LCAs & relo brackets, Panhard rod & brace, FRPP swaybars, MRT a/c H-pipe, LP OTAs, FR GT500 Touring axlebacks, FRPP diff cover, JLT PCV p/s catchcan