my two cents worth- between myself, my father(45 yrs for GM before "retiring " to build 1/4 mile monsters and my uncle(circle track nut)- we have tried K&N and found not much improvement over quality paper- and worse performance when in a dusty/dirty enviro(off pavement). But as @Rhotpursuit mentioned, most end users do not follow the instructions and end up with a oily mess.