Up to and through 00, Dex III.
01+ 3309 / T-IV (same thing; Mobil created T-IV for Aisin w/the internal code “3309.”)
T-IV is a friction modified, DexIII evolution. That’s why Dex III can be used to top up in an emergency (Aisin says so). But don’t do that unless a true emergency. The T-IV “evolved” to deal better with the higher heat and higher abuse in PWM clutches (“Pulse width modulated”).
Dex VI is technically backward-compatible with DexIII, but it’s still the wrong viscosity and shift quality is compromised. (I’ve run it in lieu of T-IV in some Lexus models as an experiment; switched back to 3309/T-IV). No reason to use it in a pre-2001, and it should never be in a 2001+ at all. Dex VI is a comparable generation fluid to Aisin WS, a low viscosity blend compromising protection in favor of minute mileage gains, and with further friction modifier alterations.
BTW, I ordered a bunch of Lexus parts (09 GS350 awd suspension rebuild) from RockAuto last week and noted they had genuine Mobil 3309 for $5.70/qt. After 5% discount.






