I dont know enough about other box's, only have rb25 and rb26 gear box exp,
But from my exp with older box's the synchro will start to fail will age and with a FULL gearbox rebuild will be back to new. HOWEVER, this is with older cars that are 40years old Not 20year old nissens.
I dont see why the synchro would fail in a 20 year old car unless its been pushed hard.