From Ford:
EFFECTIVE 12/20/2010:
Special Service Message: 21661 2011 Mustang - MT82 Cold Shift Effort
Some 2011 Mustang vehicles equipped with a MT82 6-speed manual transmission may exhibit increased shift efforts in cold ambient temperatures. This is usually most noticeable in 1st and 2nd gears but may also be noticed in 3rd through 6th gears. Engineering is investigating this concern and no repairs are recommended at this time. The use of friction modifiers or other fluid additives is not recommended since although they may provide some immediate improvement in cold shift effort, they are likely to result in long term degradation of the synchronizers.
see my post in this thread.. post #86