Toyota are very particular about the type of transmission fluid used in the Supras' A/T's, and I'd think Lexus probably are too.
The recommended type for the Supra A/T is Toyota TII or TIV (T2 or T4).
From what we could tell in the Supra, it's a Dexron based fluid with friction modifiers mixed in to Toyota's specifications.
Be forewarned, this stuff is hellishly expensive and only available from a dealer.