I have tried the dealer's fluid (expensive), pennzoil synchromesh, & Royal Purple. I have moved on to Amsoil's Synchromesh 5W-30.
My synchros are going bye-bye & so far I like the Amsoil. (2,600 miles)
With the Royal Purple I did the last 3 changes @ 5,000 mile a piece, because the trans would get hard to shift.