Castrol TranSynd 668 Full Synthetic ATF, 5 Gallon Pail

TranSynd is the full synthetic fluid developed for Allison automatics, and the 668 designation is the current Allison specification - TES 668 supersedes and now covers what TES 295 fluid did. If the unit is an Allison, this is the fluid it was engineered around, not a substitute that merely claims compatibility. Buying it as a five gallon pail is the sensible way to do it, because an Allison service is a large fill and quart bottles get expensive fast.

The reason to run it rather than a conventional ATF is drain interval and heat. Castrol rates TranSynd 668 for up to 300,000 miles or 48 months in general duty service and up to 150,000 miles or 48 months in severe duty, against far shorter intervals on non-TES fluids. It holds its viscosity under shear instead of thinning out, which is what protects the planetary gear sets and keeps line pressure where the controller expects it when a truck is pulling grades all day. The anti-shudder and clutch friction durability also hold up longer, so the lockup clutch does not start shuddering halfway through the interval.

Five gallons is twenty quarts. An Allison 1000 or 2000 series with a shallow pan takes roughly seven to nine quarts on a filter and fluid service and more with a deep pan; 3000 and 4000 series units take considerably more, so check the capacity chart for your model and pan before ordering a second pail. Change both the control main filter and the lube filter when you service it, and set the level at the operating temperature Allison specifies using the shift selector level check or the dipstick hot band, not cold.