i reccomend every 4k....this gives you a little leighway......if your thinkin about it and have time...it may get done below 4k....and if not....it still gives you time to do it before 5k if you go over.....i would not reccomend going over 5k.....this would be like your kidneys not puting good filtered blood in your body....it is what keeps your motor alive and is the single most important service..........Marc