I do think there are better things to run in a chain case then ATF. It works well because it's nice and thin and has good lube properties, but the friction inhibitors in it aren't really designed to protect a chain and sprocket, I know the manuals all say to run it, but a proper chaincase lube is better...
Cases aren't as picky as my thinking about it tho and ATF is cheap and usually helps old seals do there job! As long as oil is in there and it isn't 80weight ur good!