VWs, at least older ones, must use a GL4 transmission fluid as GL5 has sulfur compounds that can etch the brass synchros. I seriously doubt that the synthetic fluid that was installed had sulfur but if you bought the car used, you may not know what was in there before. It may have been the factory fluid as it’s not very likely that a previous owner changed the trans fluid before 100,000 miles.
Other than that, it;s worth checking the clutch actuation if hydraulic. If cable, then make sure that the automatic clutch adjuster is working correctly.
Another thing to check is the shift linkage to make sure that it is adjusted as intended.
Try the transmission forum on VWVortex.com too.
Just a few things for you to consider.