what exactly are you flushing out? what kind of bulid-up? is it carbon build up? if its carbon build up, take it to a shop, and get the fuel system flushed. they hook it up to a machine that runs a cleaner through the fuel system, and cleans out the injectors and removes the carbon
Gunk makes "Motor Flush" (I think it's gunk that makes it). It works real well. I've heard of using ATF, but I don't know how well that works.
If your oil gets dark real fast, that's often a sign of a weak PCV system, so you'll want to check that while you're poking around under there. Another thing could be lots of cold starting/short trips, i.e. chronicaly rich fuel mixture, bad rings, or overheating.
i have used tranny fluid its works ok, its has more cleaing additives in it then oil,
just let a QT of oil out then add a QT of tranny fluid drive it easily for 500 or so miles and drain it, repeat it as you see fit. at least thats how i cleaned out my dodge