Mercon Dexron III will be just fine. The AOD's hold about 11 quarts total. When I had my old stock AOD, I had problems with OD slipping and I used so much of that "tranny fix" stuff it wasnt even funny. I have to say that the Lucas Transmission Fix fluid worked really good for me for about 2 weeks, then I had to add another bottle. If your not slipping, I wouldnt see the need for it. I was going to add some synthetic to my Lentech, but synthetic can be expensive, so I just used regular dexron III. Are you going to be replacing all of the fluid or just dropping the pan? If your just dropping the pan, its only like 4 quarts (I think) that comes out.
One more thing.... stay away from that Duralube trans fluid. Ive heard some bad things about it.