Stihl VarioClean (Air-Filter Cleaner)

This is the only way to clean your air filter. Stihl has formulated a product which removes organic oil residue and returns air filters to pristine condition. The water-based product is to be applied to the Stihl filter and left for 10 minutes before rinsing it our in warm 60-degree water and leaving it to dry. 

This method is much more effective then blowing the filter out with compressed air which is likely to cause more damage to your filter then do any good.