And K&N says to clean and re-oil the outside of their air filters every 50,000 miles. They also say to use the aerosol oil spray very sparingly.
I realize that may not be practical for this crowd, but I typically go three years between air filter cleanings and never had a problem, although I don’t have EFI or a MAF sensor to worry about.
Also remember that K&N filters increase in filter rate as they collect more dust. Cleaning them too often is counter-productive.