The reason i drain and fill every 15k is it was a recommendation from the guy that does most of the work on my jeep-'his been a jeep tech for years now. He said if I wanted my tranny to last, drain and fill every 15k. His wife drives a WK with 150k on it and he does the same and no problems there. He's been dealing with my jeeps for years now and I trust him. I'm coming up on 90k so I'll be doing both filters and a drain a flush.