Ok, heres a tip from me..
this is ewhat we used to do to the rally cars and it usually worked fine !>
At first sign of synchro problems..
Drain the gearbox oil completely.
Fill with ATF - auto transmission fluid.
drive it for 2-3 weeks.. then drain and refil with recommended oil.
Often synchro problems are simply the result of glazing on the cones.. The ATF works, not only as a lubricant, but as a great flushing and cleaning fluid.
try it.. you might like the results.
Joe.