TH400 Transmission Fluid: Which Type to Use
The short answer
A stock or mildly modified TH400, also called the Turbo 400 or 3L80, is generally served by a GM Dexron-type automatic transmission fluid (ATF). Today that usually means a current licensed Dexron VI fluid, which GM states is backward compatible with earlier Dexron applications, or a quality multi-vehicle ATF that lists Dexron III compatibility. The main exception is a rebuilt or performance-built unit: if the rebuilder, valve body supplier or torque converter maker specifies a particular fluid, follow that specification instead.
Why the original specification is no longer sold
The TH400 is a three-speed GM automatic used across many cars, trucks and vans from the mid-1960s into the late 1980s and beyond in some applications. Early units were filled with fluids made to GM's older Type A Suffix A standard, and later ones with Dexron and its successors. Those earlier fluid specifications have been superseded, so owners now choose a fluid that meets a current specification designed to cover older GM automatics.
Choosing between Dexron VI and a multi-vehicle ATF
Licensed Dexron VI: the current GM specification and the most direct replacement for older Dexron fills in a stock unit. Multi-vehicle or Dexron III-type ATF: widely available and commonly used in older GM automatics; check that the label specifically lists Dexron III or Dexron/Mercon compatibility. Rebuilder or builder spec: for a rebuilt, shift-kitted or racing unit, use the fluid named by whoever built or modified it, because friction material and calibration choices can depend on it.
Some performance builders specify Type F or a dedicated racing ATF for firmer shifts. That is a build-specific choice rather than a general recommendation for a stock TH400; using it without the builder's guidance can change shift feel and clutch behavior.
Checking and filling the fluid
The usual check procedure for a TH400 is with the vehicle on level ground, the fluid at normal operating temperature, the engine idling and the shifter in Park. Add fluid in small amounts through the dipstick tube and recheck, since overfilling can cause foaming. The correct level is the one marked on the dipstick fitted to the vehicle; aftermarket dipsticks and tubes vary, so confirm they are matched to the transmission and pan in use.
Gaps to resolve before buying
No capacity figures, product test results or prices were supplied for this guide. Fluid capacity for a TH400 depends on the pan depth, whether the torque converter is drained, and any added cooler, so confirm the quantity from the vehicle's service information or the transmission builder rather than relying on a single published figure. If the transmission's history is unknown, inspect the existing fluid's condition and smell before deciding between a simple top-up and a pan service.