Lots of opinions over at Bob is the Oil Guy forum. Some swear by ACDelco, some say Pennzoil works just as well. Only two quarts drain & refill...pretty cheap to try either or both, compared to the cost of replacing the synchro.

Random Cheese brought up a good point: if the trans oil has never been changed @175K then just changing the oil might fix the problem.

 

Actually, I changed it at 60K, 120K and now I'm due again. That may well be the problem. No problems with the clutch.