i tried the ATF trick last night before i changed my oil. i poured half a bottle in there and let it run for 5-10 minutes and then drained it. it looked about as dark and nasty as it usually does. i dont know if it did anything or not.
i've tried pouring seafoam in to clean it out as well, but couldnt tell if that did anything either.
i dont think there's as much "gunk" in engines as people think, either. when i tore down the 2.9 in my b2 (it had close to 200k when i took it out), it looked like metal that had a thin layer of oil covering it. maybe that's b/c the truck was taken care of, oil changed every 4000-ish miles, i dont know. it's the only engine i've ever removed and taken apart. but of course the pennzoil guys and mobil1 folks want you to think that your engine is step away from shutting down b/c of "sludge buildup", but i dont think that's the case....most of the time.
now, the upper and lower intake manifold, the top of the pistons, those were covered in carbon build-up (as mentioned above and as would be expected)