The Southwest Research Institute did all of these tests for AEM. Bottom line: stay away from Injen filters if filtration and power are important to you and stay away from AEM's new oil-free filter if power is important to you but get it if filtration is very important to you. The K&N and Airraid filters flowed well and held a lot of dirt compared to the other 2 filters tested...but they don't filter as efficiently as some others including our OEM filters (99.8%). Unfortunately I can't comment on flow capability and dirt loading capacity of our OEM filters.
The Injen filter in link above is 5 inches long with 3" I.D., while the K&N and Airraid filters are 9" long, so take that into consideration (but it still sucks). The Injen filter flows less (new and dirty), poorly filters and clogs up quickly. The K&N and Airraid filters tested here have 4" I.D. outlets but with a reducing coupler could probably use them on the 1.8T.
This filter above is called AEM's Dryflow Cylindrical Filter Element. It is synthetic and uses no oil. It is 9" long and compared to the 9" long filters above, it filters at an extremely high efficiency (99.4%) but flows worse (new and dirty) and clogs quickly which makes it flow much worse as the dirt accumulates. Comparing Figures 1 and 2 between the two links above really shows the differences in flow (make sure you look at the x-axes since they are measured over different ranges. So this filter gives better filtration and less power. Don't have to worry about oil getting on the MAF sensor though. http://www.aempower.com/press_...id=58