'92 525I transmission fluid

[95xjsport]

I have a 92 bmw 525I and the tranny leaks a bit. I was told the tranny was sealed where only a dealer could add fluid but, under the hood I found a resevoir labled aft oil only. whats in looks and smells like tranny fluid plus it is even low. can I add transmision fluid to it? what kind? just plain atf? or does it need a specila expensive fluid like my vw jetta does?? someone let me know asap, thanks in advance...