Castrol TranSynd 668 Synthetic Automatic Transmission Fluid, 1 Gallon

TranSynd is the fluid the Allison spec sheet actually names. Castrol TranSynd 668 carries Allison Transmission approval TES 668 (the successor to the long-running TES 295 spec), which is exactly what the Allison 1000, 2000, 3000, and 4000 series automatics behind the Duramax and other medium-duty diesels are built to run on. Filling one of those units with a generic Dexron-type fluid instead of a TES-approved synthetic is the fast way to cut your service interval and lose warranty coverage on the transmission - this is the gallon that keeps the extended-drain schedule legitimate.

What you are paying for is stability under sustained heat and load. These are the fluids that live behind a diesel that tows, and TranSynd is built with strong thermal and oxidative stability so it resists the deposits and oil thickening that kill shift quality and clog a valve body over time. It also delivers extended anti-shudder durability to keep the torque converter lockup clutch smooth, extended clutch-friction durability so the pack engagements stay consistent as the fluid ages, and solid gear protection along with good corrosion and foam resistance in the cooler circuit.

On the bench this is a straightforward fluid-and-filter service item: run the engine to bring the transmission to temperature, drop and clean the pan, install the correct Allison filter kit, and refill with TranSynd, then set the level hot per the fill procedure. Because it is a TES 668 synthetic, it supports the long Allison drain intervals rather than the short ones a non-approved fluid forces. Confirm your unit calls for a TES 668/TES 295 fluid before you pour. Ships from the USA.