This is from the August 2000 Dirt Wheels Magazine page 20
"Many racers and dune riders use K&N filters, which differ
from foam units. These filters consist of layers of fiber,
sandwiched between two wire screens and pleated for more
surface area. Oil is also applied to this type of filter. These
types of filters aren’t recommended for real dusty or wet
conditions, which are better handled by the foam units."
Taken from Dirt Wheels Web Page