For RP3600, RP5500, RP6500 E, RP7500 E and INV2000 models:  Remove the filter from the housing and use a nonflammable solvent or a mixture of household detergent and warm water to wash the air filter.  Rinse the filter thoroughly to remove all of the cleaning solution.  After the filter has dried, pour a small amount of engine oil onto the filter.  Then squeeze the filter to distribute the oil throughout the filter and remove any excess oil.  

IMPORTANT: Do not wring out the filter. Wringing out the filter may cause damage.

For the RP12000 E: These models have both a foam and paper element.

To clean the foam element use the same procedure using a nonflammable solvent as described above for the other RP Series models. However, do not add engine oil after cleaning this filter. Engine oil would damage the paper element of the air filter. 

To clean the paper element, knock the dirt off by tapping the element against any dry hard flat surface such as a work bench or table top.