My 6 speed JDM gearbox is crunching when changing into 4th (at high speed only). I'm presuming it's a worn sychroniser ring? but am no expert. the part itself is cheap but anyone like to hazard a guess how much it would cost to have gearbox out and get the part/parts replaced?
Also, is 4th a single, double or tripple synchro.

Anyone had this problem?

cheers

Edited to say: this has only started happening since recent gear oil change. Immediately after the oil change i noticed that the gear change in all gears was also harder/notchier.
gear oil used was MOTUL 300 75W/90 (3 litres) - this info is taken from the service invoice

 

Update. Have checked gear oil level and it's just above the full mark on the dip stick, so the '3litre' on the service bill must be a miss print.?