Hot tub filters should be replaced annually. http://www.hottubworks.com/HotTubSpaFilters/HotTubSpaFilterReplacement.php
Cartridge pool filters are made of polyester cloth or corrugated paper. The filter can be washed with a hose and should be replaced after up to 8 years.
The filters are what needs to be replaced most often on the air conditioning units.
Most filters can no longer be cleaned but must be replaced. Older types made of a different material could be washed but not these.
Every tractor is different, and the most commonly replaced part will vary depending on the make and use of the tractor. However, commonly replaced tractor parts include air filters, spark plugs, belts and wheels.
Commutator and brushes.
I would have to say that anyone who camps would buy a tent so it is a frequently purchased item, but those who camp all the time would need other accessories that need to be replaced more often then tents, making those items the most frequently purchased.
look along the frame below the drivers door. if not in that area than you should have small inline filters in the engine compartment. most have both types of filters.
Unbleached recycled filters are the best.
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more frequently, most frequently
more frequently, most frequently
A yellow filter would be the most effective choice.