Using the air filter indicator (if equiped) is the correct way of determining when an air filter needs replaced. You can not go on looks alone or the amount of dirt that you tap out of the filter to determine if the filter needs replaced. A certain amount of dirt is required in the air filter before it reaches it's maximum efficiency. As others have stated, sometimes more dirt enters the engine just by removing the lid and taking the filter in and out. The filter manufacturer's say to replace and not blow out dirty filters. Some readers may get several years out of an air filter and others 2-3 months depending on amount of useage, conditions it's operated in, and type of air filter. Regarding the original question, the Donaldson or Filterminder would both work well and they could be found in a salvage yard for trucks-equipment.