Are the Fram filters longer and not as wide? That could be the reason why. OEM ones are probably shorter/fatter which is why it doesn't decrease the pressure like the Fram ones does...

 

in my gs-r i only used oem(sometimes s2000 oem, but didnt see an op increase like others did), and denso, which is really really close to oem contruction/quality, just cost less. Other then that, I'd only go with wix, or a k&n gold 1004(not great but it'll get you by in a pinch). Fram fuckin sucks for honda, period. maybe even for all jap applications?