If you’ve got a BMW 800 or 1200, or a KTM 1090/1190/1290, then you need  a Guglatech Rally Raid Shield 4 Air Filter.  This is the world’s first WET/DRY air filter. 

Used DRY, it’ll filter out contamination as small as 30 microns. 
Used WET, with a light coating of air filter oil, it’ll filter down to 10 microns. 

It can be washed in the field with soap and water, then re-used over and over again.  That means it lasts for a lifetime.

Read this article to learn more.  Guglatech Rally Raid Ultra 4 Air Filter 11-27-18