I have the 4l60e, it's mainly a daily driver very little towing. 15k you say, no flushes for me, I know about half the fluid gets stuck in the tc, unfortunately I don't have a drain plug on my transmission so if I'm going to swap fluid might as well do the filter, just undecided on the interval. Owners manual recommends 100k normal 50k severe, gm doesn't recomend flushes. I'm not going to do 50k intervals seems a bit to long, I go through tons of stop n go traffic, and right now in Texas it's hot, any more suggestions, help me out guys!