Most commonly, 5th gear grinding seems to be due to the synchros wearing out. If your powertrain warranty is still good and Nissan agrees it is synchro wear, they'll give you a new transmission since they don't service the internals.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |