300HP and requires filter replacement every 30K miles, but if your Z is making 400HP, you are making roughly 30% more power (when using WOT) so I would feel safe in saying you need to replace it 30% more often as you will be putting about 30% more fuel through it per mile. Granted, this is a gross approximation, but I think you follow what I mean. Filters are inexpensive so price isn't really an issue anyway.