Replacement fan ReadyNAS nv+ v2

3-pin fans vary the speed by varying the input voltage.  PWM fans vary it by sending a separate pulse that cycles the power on/off quickly.  While most PWM fans' speeds can be varied by varying the input voltage like a 3-pin fan, they may have a higher stall speed using that method than a fan intended for 3-pin control.  ReadyNAS typically do have the capability to adjust the minimum speed (via SSH) to prevent stalling, but I'm not sure how to implement it on an NV+ V2.

The NV+ V1 has a fan control "learning mode", where the V2 does not.  Together with a fan capable of only half the CFM of the original, that could lead to some "searching" for the proper speed when the temperature changes.  I don't recommend anything less than half the original's spec.  

Unless somebody does have  a recommendation of one they used in a V2, the only way to tell for sure is to try it.  I suggest you run tests in both environments to insure it does the job in the warmer one and doesn't stall in the cooler one.  Then, please report back here on your results.