Flow rate DOES remain constant, only pressure changes. When the filter elements are to resistive the bypass valve opens up to keep the pump from destroying the elements. Flow rate might drop slightly, but not significant. Only pressure changes. The oil pump is driven directly by the engine, and of a design that it always pumps a certain amount per RPM. No paper filter is going to battle that. In higher performance engines oil nozzles that require high pressure are installed in the crank to spray oil everywhere like a garden hose. They don't reduce flow rate, just increase pressure. Like trying to stop the flow from a garden hose with your thumb.

The only effect I can see from a K&N oil filter is reduced filtering performance.

If you were to compare flow rate between different oil filters it would be the same, because any filter that could actually reduce flow would have to be constructed of steel, not paper or cotton. A filter is made of paper in 60+PSI environments because that pressure is almost the same inside and outside of the filter. The pressure differential is there, but minor. Possibly not more than 5-10 PSI. If that filter because to restrict the flow of oil, the pressure will climb until the point where the oil filters bypass valve will open. During all of this, the flow rate remains the same because the additional pumping power comes from the engine, which I'm sure barely notices.

http://www.mazdastuff.com/product.cfm?ProductID=2168

the O/E filter and both o-rings for under $5.00 and a filter wrench that fits perfect

FYI, DJ

My Ford dealer who sold me the car does "first oil & filter change free".
Back in 2005, when I took mine in, they didn't have the filter in stock, and had to send someone cross-town to get one from a Mazda dealer.
My oil-change took 2.5 hours. But it was "free".

I when to 8 different car part places no one had one they could order one from $45 to 65 dollars ford was $65. Auto zone had the lifetime K&N for $53.00. Who would buy anything else regardless for air flow being able to clean saves you money on the first cleaning. Ill be honest not sure if having the K&N helps my car out but after changing my car to full syntetic oils and having the K&N air and oil filters I went from 31 to 34 MPG on my 06 FEH it all about the fuel economy.