Nissan FS6R31A

Nissan's six-speed rear-drive manual — the FS6R31A, in the 370Z and the Pathfinder R51.

⚑ The RS6F51 is not this gearbox, and the letters are what say so: FS is longitudinal, RS transverse. The front-drive six-speed of the Sentra SE-R Spec V and the Maxima has its own page here. A parts listing that says only Nissan 6-speed manual covers both, and they share nothing.

The FS6R31A is the one people modify, and its failure follows: third and fourth gear synchros under hard use, then the gears. A 370Z that grinds into third under load is at the end of its rating rather than badly maintained — the same story as the Tremec T-5 and the Aisin AY6.

Nissan's SynchroRev Match is a control feature, not a gearbox one. It blips the throttle on a downchange, and when it misbehaves the fault is the clutch position sensor or the switch, not the transmission — which is worth knowing because the symptom feels entirely mechanical.

It takes a specific Nissan gear oil, and a heavier conventional oil makes it notchy when cold. That symptom has now appeared on seven gearboxes in this catalogue across seven marques, and it is never the synchros.