Scrubbing the synchros for better shifting
 

Long story short, I swapped out for better gear oil because my 6th gear synchro was grinding instead of engaging smoothly, except when I shifted very lightly and slowly, but the better gear oil didn't do a damned thing to improve matters. On the drive home, though, I remembered something I did back when the 3rd gear synchro was giving me trouble, and as I was sitting at a stoplight I tried to engage 6th gear without opening the clutch first. With only a moderate pressure, the blocker ring stopped doing its job and the lockring tried to engage on the side of the gear, producing the expected grinding sound. So I did it again, more lightly, and held it there for a brisk count of 5, then let the shifter go back to Neutral to let the synchro cool down. I did this a few more times, using progressively firmer pressure on the shifter, until I was able to push hard on the shifter without the synchro trying to lock in place.

So whether there was residue from the old gear oil or some other kind of gunk that was keeping the 6th gear synchro from contacting properly, scrubbing the friction surfaces together seems to have helped. Time will tell if the fix works for the long-term, though. I'm curious to know if anyone else has ever done such things to "repair" a synchro that was exhibiting signs of insufficient friction on the friction surfaces.

Fortunately, there's no issue with my synchros as of now... And I hope it continues that way. I have to admit, grinding gears is a terrible feeling. Its happened to me about 3 times in 7k miles of ownership, and each time was due to my own driving error. Don't knock me for it, I'm only human. ;)

My 6th gear synchro was doing it every time I shifted once the gear oil heated up, except when I shifted very gently, taking a couple seconds to complete the shift. I'm not happy to admit it, but I kinda got used to it after a while, though fortunately I didn't do much shifting when I was going fast enough to need 6th gear in the first place. When I got the gear oil changed, the oil in the gearbox looked a shade darker than the oil in the diff, so knock on wood, I don't think there was any damage done except perhaps rounding-off the edges of the splines.

I suppose the 6th gear synchro is in a unique position, in that the only job it ever does is to slow down the input shaft when upshifting and never to speed up the input shaft when downshifting, so the friction surfaces only ever get scrubbed in one direction. Maybe that unique wear pattern explains why only the 6th gear synchro was acting up when the others were fine? Or is there something different about its construction?

Why don't you bring your car in for a warranty check? There's a story of some s2 transmission being bad from the factory, although it was a 5th gear issue if my memory serves me well.

Honestly? Because I don't feel like listening to them try to tell me that my choice in gear oil is solely to blame for the transmission's quirks. Before I upgraded the gear oil, 1st and 2nd would grind when the oil was cold; after upgrading the gear oil, 5th and 6th would grind when the oil was hot. I think what it really comes down to is the graphite-coated-steel friction surfaces take a lot longer to break-in than the brass ones most people are used to, which is why I wasn't too scared about scrubbing the friction surfaces for several seconds at a time while I was sitting at stoplights.

I drove the car today, more than enough to get the gear oil hot, and 6th gear behaved fine. My only complaint about the gearbox at this point is I have to push the shifter a bit further to the right than I'd prefer when upshifting into 3rd, but that's not a problem so much as something I'll continue to get used to.

it can happen when things get jammed between the syncros.

Not sure I want to ask, but what can get jammed in there? There aren't exactly any loose parts as far as I know.