Due to the fact that my truck is starting to idle rougher since all the stations around here have now got ULSD, I've decided to go with B20 to help do away with the rough shaking idle(bad thing is it's at the already expensive stations so even though B20 is cheaper compared to their no. 2 diesel, it's about the same as the other stations that are cheaper). Now here comes the question, I know about every 2 to 3 fillups, I should change the fuel filter, however, one of my 2 fuel filters is a FASS filter, so how often should that one be changed out? Should I maintain the double life span of it's changes so after every 4-6 fill ups swap out the FASS fuel filter or should I still do the 2-3 fillup scenerio?