I recently changed (most of) the trans fluid and went for some Castrol Dexron II equivalent from Opie Oils. Dexron II was the recommendation of GM, but that has now been replaced, first with IIE and then III. But, apparently, Dexron VI is a suitable replacement and can be used.
My only proviso was that I wanted to use a "known" brand, as opposed to something form one of the supermarkets or auto stores.