Both my trucks had the oil type K&N.
Notice I mention had. I returned to the paper filter. During the service on my newly acquired 2000, I noticed dust on intakes. The reusable oil type filters in our stock airboxes leave something to be desired. In the dusty ag conditions which the trucks are exposed to, this is less than satisfactory.
I think the next filter change on one of the units I may try the 6637 modification.