Whether you change it at 6 or 15k, the filter is still trapping particles with the same efficiency. You are simply throwing away money, until it starts to clog and restrict performance. The size of the holes in the filter media do not change with age. I'm not saying I recommend leaving them in till they clog, however changing them any earlier than that will do nothing for you. If you're worried about fuel filtration, add a 2 micron polishing filter in line .