I used a siphon pump a few weeks ago and took out about 6 quarts and topped back up with new fluid. It wasn't due for tranny service yet but I got a great deal on ATF+4 and thought I would freshen things up because of a few wierd shifts recently at WOT. When I do drop the pan though I will use the siphon to drain as much fluid as possible. I tested the possibility of draining the oil this way but the last few bends in the dipstick tube prevented the drain tube from hitting the oil pan. I will however use the siphon for the diffs when they are due.