I believe it's denim paper, kind of like what dollar bills are made of. It can be washed and reused, but not oiled. I'd still go with the K&N replacement, as it is superior in every way!

 

"Backplay" sorry but you are wrong! It is not paper, and it is washable. "Washable, but do not oil". I've got 2 years 20k miles on mine, wash it every service, looks new. I may replace it this year just for piece of mind.