Oiling the outer filter is going to cause more problems than it helps for exactly the reason posted in the pictures. these motors don't suck that hard that we need 3/4 of an inch of oiled foam to catch all the dirt. The mbx5 came with a single stage filter and they worked fine, a lot of people still use the ofna universal filter elements which work too. The purpose of the outer filter is the keep the inner one from caking up and causing a clog. The pre-filter helps keep the big stuff from sticking and fall off for the most part.