TH350 Transmission Fluid: Which ATF to Use
The short answer
A stock or mildly rebuilt TH350 (the GM Turbo-Hydramatic 350 three-speed automatic) is designed for GM's Dexron-family automatic transmission fluid (ATF). The original Dexron grades are no longer sold. In practice, owners use a currently licensed Dexron fluid, which GM states is backward compatible with earlier Dexron applications, or a quality multi-vehicle ATF that the maker says is suitable for older Dexron specifications.
One condition can change this answer. If the transmission was built with aftermarket clutches, a performance valve body or a shift kit, use the fluid the builder or parts maker specifies. That instruction takes priority over the general recommendation.
Why a Dexron-type fluid fits
The TH350 was a GM transmission, so its friction materials and seals were matched to GM's own ATF standard. Dexron is a friction-modified fluid, meaning it contains additives that help clutches engage smoothly instead of grabbing. Using a fluid built to that friction profile keeps shift feel and clutch wear close to what the original design intended.
Choosing between current fluid options
Current licensed Dexron fluid: the most direct modern substitute for the original GM specification in a stock transmission. Multi-vehicle ATF: acceptable when the label specifically lists suitability for older Dexron applications. A vague claim of compatibility is not enough. Builder- or kit-specified fluid: the right choice for a modified or performance-built unit, whatever the general guidance says.
Some owners debate whether a newer, thinner Dexron grade suits an older transmission as well as earlier formulations did. That debate depends on condition and use. No test data was supplied here to settle it. In a modified or heavily used unit, the builder's guidance is the safest way to resolve it.
Fluids to avoid unless specifically instructed
Type F ATF has a different friction profile from Dexron. It was intended for transmissions that needed firmer clutch engagement. Some racers deliberately use it to sharpen shifts, but in a stock TH350 it changes how the clutches engage. Use it only if the person who built the transmission recommends it. Also avoid fluids marked only for continuously variable transmissions (CVTs) or dual-clutch transmissions, since those are formulated for different designs.
Checking the level correctly
Automatic transmission fluid is usually checked with the engine running and the fluid at operating temperature, with the vehicle on level ground and in the position the dipstick or service manual specifies. Overfilling can cause the fluid to foam, and underfilling can cause slipping. Both can mislead you when diagnosing shift problems.
Capacity: an information gap
No verified capacity figures were supplied for this article, so none are given here. The amount needed differs between a pan-drop service and a full refill after a rebuild, because the torque converter holds a share of the fluid. Pan depth also varies with deep or aftermarket pans. Confirm the figure in the service manual for your vehicle's model year and installation, or with your transmission builder. When refilling, add fluid gradually and check the dipstick as you go.
Next step
For an original or standard rebuilt TH350, buy a currently licensed Dexron fluid or a multi-vehicle ATF that explicitly covers older Dexron applications. For a performance-built unit, check the build sheet or ask the builder before buying anything.