TranSynd 668 Full Synthetic ATF 3 Gallon for Allison 1000

This is the three-gallon pack of Castrol TranSynd 668, the fluid Allison actually names on its own spec sheet. TranSynd 668 carries Allison Transmission approval TES 668, which supersedes the long-running TES 295 spec, and it is what the Allison 1000 and 2000 series six-speeds behind the 6.6L Duramax are built to run on. The same fluid covers the medium-duty 3000 and 4000 series. Fill one of those units with a generic Dexron-type ATF and you cut the service interval down and give up the extended-drain schedule the truck was sold with.

Three gallons is the size that matches the job. A deep-pan Allison 1000 spin-on service takes right around 13 quarts, and a shallow-pan drain and fill runs about 7 to 8 quarts, so a single gallon leaves you short and a case is more than you need. This pack is 384 fluid ounces total, three one-gallon bottles, which covers a full pan-and-filter service with fluid left to set the hot level.

Doing the service: run the truck until the transmission is at operating temperature so the fluid drops out clean, pull the pan, replace the internal filter and the spin-on if your unit has one, reseal the pan, then refill through the fill tube. Bring it up short, run it through the gear ranges, and set the final level hot using the Allison fill procedure rather than pouring in a fixed number of quarts. What TranSynd buys you over the drain interval is thermal and oxidative stability, so the fluid does not thicken and varnish the valve body under sustained towing heat, plus extended anti-shudder durability that keeps the converter lockup clutch from chattering as the fluid ages. Confirm your transmission calls for a TES 668 or TES 295 fluid before you pour.