The law says that you don't have to send in a warranty card on any product in order to have the warranty. Most warranty card questions are just for advertising purposes, after they tabulate the results the card goes in the trash, no record is kept except by junk mailers and telemarketers. Warranty cards that are to be kept for a permanent record must be filled out by the vendor who sells you the product.
K&N will want a copy of your receipt though, they are very good about replacements.