Don't get me wrong, I probably could run my NGKs alot longer, I am just psyco <img src="/forums/images/graemlins/scared.gif" alt="" /> about preventitive maintanance. I change my coolant, trans fluid ,gear oil in my xfercase and diffs once a year. My oil every 3mo (1500mi)w/Mobil 1 synthetic. Pack my wheel bearings every year, bleed my brake fluid every year. You get my point. The changes may not be nessesary, but I don't like surprises. Everything gets checked after a long trip and wheeling. Why do I do it? Because I can, and fluids can tell a story.