I would be careful. Synchronizers need friction to work, you put a
synthetic such as red line or royal purple and you might not have the friction you need for the synchronizers to work properly. It always amazes me that "some guy" or "my mechanic" always knows more than the Orignal equipment manufacturer, wow they must be stupid. Use the gm synchromesh as stated and if you still have a problem then it is component related not fluid. I had a 3550 and it was the smoothest shifting transmission I have ever owned, I used GM synchromesh.