I provide this only with the deepest respect for the opinions of knowledgeable fellow Beavers. But feel I would be negligent not to put forth the unpopular alternative that the latest generation of K&N diesel filters are different. Their design now uses a hybrid filter medium of a thick "lofted" non-woven synthetic media co-pleated between 2 layers of powder-coated aluminum “wire”. You no longer have to deal with a messy oil coating to wash off and reapply. Just spray on degreaser and blow or hose out or powerwash from the inside. Admittedly it’s probably still understandably more palatable for many to just front the cash repeatedly for a new filter.
When I put to Bob Henderson (Henderson’s Lineup in Grants Pass) the strong concerns previously related here in older threads, he responded with a reassuring confidence in the latest K&N product. Bob is definitely one whose opinions regarding anything chassis is to be seriously taken. I had the old style K&N on my previous coach and had no issues despite many off highway endeavors. The only hassle was cleaning and reoiling a couple times... the oil made things messier.
All that said, I can’t disagree with using any of the other stock filters recommended in this thread. They will (initially at least) be less expensive anyway, all other parameters aside. I just felt that all viewpoints should be represented, even this perhaps less popular one. I know there are one or two others here that are into race cars, knowledgeable about those air filters and turboed engines, and would, like Al, back the option. Nevertheless, out of deep regard for my C9 I’d still not likely get myself into a situation where I had to low gear wide open up a steep, dirt backcountry road, regardless of which filter I had.
Joel