I've done one drain-and-fill session, and will do at least two more to make sure I've flushed most of the old black fluid that's in there now. At what point should I change the filter? If I put a new filter in now, then it'll have all that mucky old fluid running through it until I'm finished with all the drain and fills. I figure I need to drive at least a little bit in between swaps to make sure the old mixes thoroughly with the new. So should I wait till the last drain and fill and then put the new filter on?