The original ATF spec called for Dexron-III, didn't it? Or even if it called for old Dexron-II or IIE (I don't know when Dexron-III replaced Dex-II), use any brand of ATF that says anything about Dexron-III on the label. I feel that the name brands are better, and that synthetic will live longer.
Dexron-III in no longer "officially" produced, but is still widely made under other names. If the labels says anything like, "recommended where Dexron-III was originally used," that's what you want. Do not use anything that does not refer to Dexron-III on the label.