Contaminants in modern gasoline will eventually restrict the fuel filter. This can lead to drivability symptoms such as poor passing power, long crank times and others. Also a restricted fuel filter puts more strain on the fuel pump and can lead to more costly repairs . The better quality gasoline we run the longer a fuel filter will last. Since we may never know how good or bad the gasoline we pump is, we recommend the fuel filter be replaced on a regular basis of about every 30,000 miles. Contaminants in modern diesel fuel will eventually restrict the fuel filter. This can lead to drivability symptoms such as poor towing power, long crank times and others. Also diesel fuel is very susceptible to water contamination. This can lead to algae forming in the tank. Any contamination in diesel fuel can lead to very costly injector damage. Usually the more fuel a station pumps the better quality the fuel is. We suggest to use a "high volume" station for this reason. We also recommend the fuel filter be replaced every 15,000 miles.