The Allison TES 668-approved full synthetic ATF for Allison 1000/2000/3000/4000 automatics behind Duramax and other diesels, giving the long extended drain intervals the factory calls for. One gallon.
A single gallon of Castrol TranSynd 668 is the size most Allison owners actually need between full services. If you run a 2001-2019 Silverado or Sierra 2500/3500 HD with the Duramax and the Allison 1000, you are probably here because the level is low on the hot check, a cooler line weeped, you just spun on a new filter, or a shop handed back the truck with a fluid you do not recognize. This gallon is the Allison TES 668 approved full synthetic, so whatever you add matches what the transmission was built to run on.
In the Allison 1000 the fluid is not just lubricant. It is the hydraulic medium the TCM uses to apply the clutch packs through the trim and shift solenoids, it carries heat out to the cooler, and its friction properties decide how the converter lockup clutch engages. TES 668 is the spec Allison introduced in 2020 and now recommends across the 1000, 2000, 3000 and 4000 Series. It is backward compatible with TES 295, TES 389 and TES 468 fluids, so you can add it to a unit that was filled with any of those without a flush.
Allison's interval table treats anything less than 100 percent TES 668 or TES 295 as a mixture, and a mixture has to run the much shorter TES 389 change intervals. Topping off with a generic multi-vehicle ATF quietly moves a truck from the long 150,000 mile general-duty schedule to the 50,000 mile one. Keeping a gallon of the correct fluid on the shelf is the cheap way to avoid that.
Check the level the Allison way before you add anything: warm the sump to 160-200 F, level ground, engine idling in Park or Neutral, and read the stick twice. The HOT band is only about 1.06 quarts top to bottom, so a single gallon covers most corrections with plenty left over. If the fluid is burnt or there is metal on the filter magnet, adding fluid will not fix it; work through our Allison 1000 shudder diagnosis guide or the limp mode guide first.
This fluid is for any Allison 1000 or 2000 Series automatic, which on pickups means the 2001-2019 Chevrolet and GMC HD trucks with the Duramax diesel or 8.1L gas engine, plus Kodiak and TopKick medium-duty trucks. It is also approved for the 3000 and 4000 Series. One gallon is four quarts, so it is a top-off or filter-change quantity, not a full service. A shallow-sump refill is about 7.4 quarts and a standard-sump refill about 10.6 quarts, so for a pan drop buy two or three gallons, or step up to the three-gallon pack. For other parts that fit your truck, browse the Allison 1000 parts page.
Interval and capacity details for every sump are in our Allison 1000 fluid and filter service specs.
| Type | Full synthetic automatic transmission fluid |
| Approval | Allison Transmission TES 668 (supersedes TES 295) |
| Volume | 1 gallon |
| Fits | Allison 1000/2000/3000/4000 series automatics (Duramax and other diesel applications) |
| Drain interval | Supports Allison extended fluid change intervals |
| Stability | Thermal and oxidative stability to resist deposits and thickening |
| Durability | Extended anti-shudder and clutch-friction durability |
| Before use | Confirm your transmission specifies a TES 668/TES 295 fluid |
| Ships from | USA |
Yes. TES 668 is backward compatible with TES 295, TES 389 and TES 468 fluids. Just know that unless the unit ends up 100 percent TES 668 or TES 295, Allison treats it as a mixture and the shorter TES 389 change interval applies.
No. A shallow-sump refill takes about 7.4 quarts and a standard-sump refill about 10.6 quarts, before external lines and cooler. One gallon is for top-offs and spin-on filter changes. Buy two or three gallons, or the 3 gallon pack, for a pan service.
Do the hot check: sump at 160-200 F, level ground, engine at idle in Park or Neutral, and read the stick twice. The level should sit inside the HOT band, which spans only about 1.06 quarts.
Only if low or degraded fluid is the cause. If the fluid is burnt, there is metal on the filter magnet, or codes are set, diagnose the converter clutch, solenoids and valve body before spending money on fluid.
Prognostics requires an Allison-approved TES 295, TES 668 or TES 389 fluid and does not allow mixtures. TranSynd 668 meets that requirement.
