Transynd 668 Transmission Fluid - x3 One Gallon Jugs (TES 668). Castrol TranSynd 668 has been specifically formulated for Allison Transmission on-highway automatic transmissions. This premium, full-synthetic fluid is Allison TES 668-approved and it builds on the demonstrated performance of the previous Allison TES 295 spec with key differentiators that include improved wear protection, extended anti-shudder durability, and extended clutch-friction durability.