There really isn't much need to further filter transmission fluid, it is a closed system. Like DP stated, heat, not dirt is what kills automatics. As long as you don't overheat it, many manufactuers these days leave auto transmission's sealed for life, no need to change ever. The big three truck builders have us changing pretty often, knowing the trucks are getting worked and over worked in many cases. Keeps warranty claims down not only by keeping the fluid fresh but they will deny you if you do not change at the prescribed intervals.