In regards to replacing synchros in the STi 6 speed, when ordering the part(s), order the synchro set for a MY06 STi, as it has Carbon-reinforced synchros on 4th, 5th and 6th gears. These can be found through a few online vendors that typically carry tranny parts, or obviously the dealer.
I personally run Motul tranny fluid for the 6 Speed (and Rear Diff), and have preferred it over the stock Subaru fluid in the roughly 10,000 miles I've used it for; we'll see how the Motul lasts. In talking to a few shops, many have had success with cars that had a grinding issue (possible synchro failing), and switched to run 1/2 Redline Shockproof fluid and 1/2 Motul fluid for the STi 6 speed transmission.
