Guys, before you ruin your bikes completely you should get used to the fact that proper shifting per se has nothing to do with a working or not working clutch. Constantly airing or UV exposition of the the box' internals won't make anything better. These parts like the darkness - that's something I was told nearly 20 years ago and this was one of the best advice I ever got. If your gearbox is broken then it's time to get into it, never before that or just out of curiosity.
Instead make sure the linkage has enough play, make sure the levers are parallel to each other, make sure they are centered (same movement to the left and right) and finally make sure they're lubed. A stuck gearbox when hot in most cases is just a matter of sticky leverage, not of air in the clutch line or the wrong colour/manufacturer of the oil and other ideas like that.
The six-speed box is a very good one. It's light, quick shifting and totaly reliable. Even when on some bikes "the spring" (and it's boss) are worth some attention
hubert