Don't put anything but ATF in them. They're designed to be run with ATF.....use it. I personally run Redline D4, which is a synthetic ATF designed for manual transmissions that require ATF.
also make sure you remove the fill plug first (before draining). otherwise you can end up with an empty trans or not know exactly that you have it filled to the proper level (and wont be able to check the level in the future).