4000 plus hours on our 6.3544's with no air filters and they run as good as the day they were new, and they don't use any oil between changes either.* Dirty air can wear out the valve guides, in turn leading to oil consumption and smoking. For us, so far, so good. If your concerned about service intervals, you can purchase a tell-tale vacuum gauge that is installed in the hose between the filter and the intake.* When the vacuum in the hose reaches a pre-determined level, (normally under load when air flow is higher) the indicator goes from green to yellow to red, and stays that way until you re-set it. A clogged filter will cause a decrease in power, and increased smoking. * Far less expensive, and easier too might simply be to clean the filter if you are in doubt.* Take a close look to see if there is noticeable debris in the pleats of the material, and if so, try blowing it out from the inside of the filter with a gentle stream of compressed air.* Depending on the filter type, you may need to wash in soapy water and re oil.* All in all, either way, it's no big deal...................Arctic Traveller