The K&N 33-2399 is a reusable panel-style replacement element for 2007–2009 Nissan 350Z and Infiniti G35, G37 and EX35 models. Its flexible frame holds layered, oiled cotton gauze media that the manufacturer says is designed for high airflow with strong filtration, and it is rated up to 99% efficient. At 7.125 x 6.625 x 1 in (181 x 168 x 25 mm), it is sized to sit in the factory airbox, so you keep the stock intake layout and sensor position.
The reason to pay more than a disposable paper filter is longevity. Because the element is washed and re-oiled instead of replaced, one purchase can cover many oil changes' worth of service. The manufacturer backs it with a No-Hassle Lifetime Limited warranty. Owners on user forums report that the power gain over stock is subtle and often unnoticeable, so buy this for reusable filtration and engine protection, not for a dramatic horsepower jump. Searches worth targeting: 350Z drop in air filter 2007 2008 2009, G35 G37 reusable cotton gauze air filter, washable Z33 intake filter, and K&N 33-2399 vs stock airbox filter.
The manufacturer lists fitment for 2007–2009 Nissan 350Z and 2007–2009 Infiniti G35, G37 and EX35 models. If your vehicle falls outside those years, confirm fitment with the manufacturer before ordering.
The manufacturer specifies a re-oiling amount of 0.61 oz (18 ml). Owners on user forums warn that over-oiling can foul the mass airflow sensor on these cars, so apply oil sparingly.
The manufacturer describes the filter as designed to improve horsepower and acceleration, but no horsepower figure is published. Owners on user forums report little to no noticeable change in performance compared with the stock filter.
This is a drop-in design: remove the stock paper element from the factory airbox and seat the K&N in its place. Before installing, wipe out the airbox so debris does not reach the intake. Owners on user forums caution that over-oiled K&N filters have caused MAF sensor problems on these cars, so re-oil lightly and let excess oil drain before installation. Confirm the exact airbox access steps and any required tools against the manufacturer's installation instructions, since those details were not confirmed in my research.
