'Bob - The Oil Guy" did some air filter testing and I thought it was an interesting read. Not sure exactly how old the testing is, but - I doubt filter technology - or the different brands - change too much over time. As he says, paper is paper, foam is foam, and cotton is cotton.

The nice thing is that even though he was testing using a Miata, the Miata uses the same style flat rectangle filter the RX-8 does, and therefore the results would be argueably equivalent.

Don't want to spoil it for those that read all the way through - but once again - someone we know well has done their homework and not provided much if any wiggle room when all is said and done.

http://www.bobistheoilguy.com/airfilter/airtest1.htm

 

If you hold up a K&N filter to the sky, you may be surprised that you can actually see daylight; there are holes that big. Its kind of disquieting. That being said, I ran nearly all of my 150,000 miles on my '91 Miata on a K&N filter. I'd guess if there is a practical difference, it's pretty small. I did realize somewhere around the 3rd time that cleaning and re-oiling the filter is somewhat a pain in the ***. I've decided that I much prefer popping in a fresh new paper filter when it needs it. That, and the potential for mugging up the MAF, make the K&N not worth it to me.

 

as whenever this wonderful test is brought up, bob never really checked the catch filter to determine the amount of oil on it, only "it didn't look like oil"... and a loss of oil from the filter would show up as a drop in "filter efficiency" of his great test

he could have weighed the amount of dirt he put it, weighed the amount that got through to the final filter, and weighed the amount the air filter picked up and reconciled the difference... but then that would be an unbiased test and just not his style