For most 2008 GMC Sierras with the common gas engine, the recommended transmission fluid is Dexron VI automatic transmission fluid (ATF). If your Sierra uses the Duramax turbodiesel with an Allison 1000 transmission, you should use Allison TES-389 ATF. Always confirm the exact specification in the owner's manual or on the transmission dipstick.
Two main transmissions available in the 2008 Sierra
The 2008 Sierra lineup includes both gasoline-powered models typically equipped with the 4L60-E automatic transmission and the heavy-duty Duramax diesel models paired with an Allison transmission. Each transmission has its own fluid specification, so it’s important to identify which one your truck uses before buying fluid.
Gasoline versions: 4L60-E automatic
GM’s guidance for the 4L60-E in many GM trucks is to use Dexron VI ATF. This fluid is backward compatible with older Dexron II/III specifications, but Dexron VI is the preferred choice for newer transmissions to ensure proper friction characteristics and shifting performance.
- Dexron VI Automatic Transmission Fluid (ATF)
- Meets GM specification for 4L60-E transmissions
- Backward compatible with older Dexron II/III systems, but Dexron VI is preferred
Using the correct Dexron VI ATF helps maintain smooth shifts and proper transmission longevity. Avoid using fluids not specified for the 4L60-E, as incorrect fluid can affect performance and durability.
Duramax diesel versions: Allison 1000
The Duramax-powered Sierra uses an Allison 1000 transmission, which requires a more specific fluid standard. Use Allison TES-389 Automatic Transmission Fluid. This specification is distinct from Dexron VI and is designed to meet the demands of the Allison 1000 family. Do not substitute with Dexron VI unless a GM/Allison technician confirms compatibility for your exact build.
- Allison TES-389 Automatic Transmission Fluid
- Required for Allison 1000 transmissions used in Duramax-equipped Sierras
- Do not substitute with Dexron VI unless explicitly approved by GM/Allison for your model
If you’re unsure which transmission your truck has, or if you’re considering fluid alternatives, check the owner’s manual, the transmission dipstick label, or consult a GM dealer or a qualified mechanic.
Maintenance tips: checking and changing the fluid
Regularly inspecting transmission fluid helps prevent problems. Check fluid level when the engine is warm and idling, with the transmission in Park (or Neutral for some models) and the engine running. The fluid should be red and translucent; a burnt smell or dark color indicates replacement is due. Follow GM/service interval guidance for fluid changes and use only the specified ATF for your transmission type.
When replacing fluid, use the appropriate brand and specification from the list above, and consider replacing the transmission filter if your truck’s maintenance schedule calls for it. If you’re not comfortable performing a fluid change yourself, a qualified technician can perform the service and verify the correct fluid type for your specific Sierra.
Summary
In short, a 2008 GMC Sierra with a gasoline engine typically takes Dexron VI ATF for the 4L60-E transmission, while a Duramax diesel with an Allison 1000 transmission uses Allison TES-389 ATF. Always verify the exact specification for your vehicle in the owner’s manual or on the transmission dipstick, and consult a professional if you’re unsure.