I've installed a new option for the Air Filter. I've used the K&N drop-in, the K&N Chevy Round unit, the Really Large Baldwin Round paper filter. I've used the K&N filters on my vehicles since they came out ?? years ago and never had a problem on any of my cars or trucks. But reading here about the possible issues with Turbo Diesels, I reconsidered their use on my PSD. In looking at the Large Round Paper filters, I discovered they have no inside screening to the Turbo-side of the filter. That just did't seem right, given the sucking ability of the Turbo. So I looked elsewhere. I've come up with using a Chevy Diesel OEM Intake filter system as it has almost 3X the filter surface area than the Ford OEM filter (aprox - 4230 sq." against Ford's 1560 sq.") in addition to having the metal screen to the Turbo-side. Don't know if it will allow 3X the air through, but gessing at least 2X the air flow is an improvment over Ford OEM. It also appears to have a better sealing system for the "box to top" than the Ford OEM.
To get it to fit, I had to cut all but 1.75" of the lower box off and intalled it on it's side in place of the Ford OEM box. With the bottom of the box cut off it becomes an open-air unit. Since my rig has only minimum hop-up - air intake, 5.6 to 18.5K mod, & straight piped - I figure 2X the air flow is enough. Another option to consider.