All my Honda's with 6 Speed manuals had the flakiest transmissions. Civic, S2000, Accord. Before they had the 3rd gear TSB for almost ever vehicle I ran GM Synchromesh in all of the cars and NEVER had an issue except in the S2000 on very hot days.
I believe that the right fluid can not only mask mechanical issues or defects, but is also critical protection for certain conditions.
The FRS/BRZ trans is clearly another case where in cold temps the OEM fluid is not working properly.
Ordered 3 QTs of Pent as recommended here: $84.98 Shipped. It's a bit expensive, but not being able to use second gear for 20 minutes without a grind in the winter will be worth it. I just hope the fluid holds up to 30k miles.
http://www.oreillyauto.com/site/c/de...35i&vi=1445226