I never liked the manual transaxle lubricant that Honda supplied with my Insight. It made the transaxle very notchy in the cold weather, and after a frustratingly high number of missed 1-2 shifts cold mornings, I talked my dealer into changing to a synthetic transaxle lubricant at 15,000 miles.
I had run Redline MTL in the SPG and had really liked what it did to the transaxle. (I blame the ultimate failure of the transaxle on my failure to replace all the fluid that leaked onto my driveway each night.) I asked for Redline MTL.
The dealer did some research and concluded that BG Syncroshift was the way to go. I don't know if he used the synthetic Syncroshift II, though I did ask for a synthetic. And since I only need a little more than a quart and a half, the $30 it cost me suggests I probably paid for the latter.
The difference was dramatic--much improved shifting when cold. I had the unfortunate opportunity to try out a couple of alternatives:
At 30,000 miles the dealer performed the standard maintenance, which included replacing the transaxle lubricant. They went back to Honda's MTL, which I noticed immediately and did not like. They replaced the Honda stuff with more Syncroshift, and this was good for another 30,000 miles.
At 60,000 miles, I again requested a synthetic manual transaxle lubricant. They put in Mobil 1 synthetic motor oil. This was predictably terrible, but I drove it for a few days at the dealer's insistence. They again replaced the fluid with Syncroshift, and I've been happy up to now, where at 90,000 miles I'm facing another transaxle lubricant change.
The web is not kind about either Syncroshift or Redline MTL in Honda transmissions. Because of the good success I've had with Syncroshift, plus the fact that I've been running it already, I'll stick with it so I can file a long-term report someday.
GM's Synchromesh seems to have a popular following with Honda transmission enthusiasts, though the posts of people I've seen don't seem to have as much data about how well it holds up. My experience with Syncroshift has not suggested much (if any) decline in performance over 30,000 miles.
There's probably a very interesting discussion to be had about friction modifiers, shear strength, and so on in manual transmission applications. Once upon a time I'd pretend to know what I was saying, but this business about "corrosion" and "seal compatibility" has me doubting myself. But I do know this much: I have been very impressed with synthetic MTLs in what appear to be difficult transmissions otherwise.