According to the service department at my local dealer, the G35 Coupes have two different styles of air filter. He described one as a vertical cartridge, and the other as a "non-cartridge" that is in a "clamshell" type of enclosure. My G35 Coupe has the vertical filter, and today I replaced the stock paper air filter with a K&N filter. K&N part # 33-2031-2 was what K&N tech told me would fit. The other style K&N told me was part # 33-2259. I would suggest you check which type of filter you have before purchasing. After getting the part number, I contacted my local Advance Auto Parts store, which had the K&N in stock for $44.95. It is supposed to allow for 2-3% increase in horsepower, however I have not noticed anything as of yet. Perhaps with the increased air flow, the computer may need to recalculate for a bit. Hope this is of some help to those of you that are interested.
-kj