synchro is the mechanism that match up ur gear speed in order to made a smooth shift, if synchro got problem, symptom probably like mine, when slow or low speed shift to 5th gear fast, smooth, but, if u rev high or change gear fast in a faster speed like over 80km/h to 5th gear, it can be shift but u will heard some kind of sound like "gruok" that will make u feel bei song, luckily it was no the gear grinding "krack" sound, synchro can be replace , do found out few service provider in zth, guys can search in zth.......