I've been using Castrol Transmax DEX/MERC (which is a Dex 3 spec fluid). Use it in my main truck (790 HP / 1300 lb-ft at the tires) and my Suburban (425 HP / 750 lb-ft). That's what my builder said to use for the warranty and I saw no reason to stop once the warranty was up, though they said any Dex 3 spec fluid should work.