Whatever stuff from the pan is picked up by the fresh ATF will also get dilluted out on the next drain and refill. The trick is to give the new ATF time to do its cleaning for a while whith the flush machine does not do. If you have the technical means and time to drop the pan when you do the first drain and refill by all means do it. Its satisfying to know all that stuff is out. The Lexus LS400 gasket and filter kit can be had at any NAPA store for less than $30. But I don't think its a do or die affair. Just the extra mile. These are overbuilt trasmissions designed to live a very long time with very dirty ATF. A member here has gone 270k miles on the original trasmission untill a leak in the radiator finally killed it.