DO use an oil that is formulated specifically for foam filters. Gear oil is not the same!
DO allow the filter to properly dry after washing. Oiling the filter without allowing it to dry sufficiently will trap some of the moisture.
DO pre-oil the filter at least 24 hrs before riding to allow the oil to evenly penetrate the pores and allow the thinning agent to evaporate. A recently-oiled filter will cause rich conditions.