2018 GMC Sierra 1500: Transmission Fluid & Service

Quick answer

• Transmission make/model: General Motors 6L80 (RPO MYC) or 8L90 (RPO M5U)
• OEM fluid spec: DEXRON VI (6-speed) or DEXRON HP (8-speed)
• Drain & fill capacity: Approx. 6.0 quarts (6L80) or 8.5 quarts (8L90)
• Full overhaul capacity: 12.4 quarts (6L80) or 11.0 quarts (8L90)
• Filter OEM part number: 24254203 (6L80 pan filter), 24274402 (8L90 pan filter) — verify with dealer or service manual
• Recommended service interval: 45,000 miles (severe service) or 90,000 miles (normal service)

Fluid type explained

The 2018 GMC Sierra 1500 was available with two different automatic transmissions, the 6-speed 6L80 and the 8-speed 8L90. It is critical to use the correct fluid, as they are not interchangeable.

• 6L80 Transmission: This transmission requires a fluid meeting the DEXRON VI specification. DEXRON VI is a synthetic blend fluid that offers excellent oxidation resistance and consistent shift performance over a long service life. It was introduced in 2005 as an improvement over the previous DEXRON III fluid.

• 8L90 Transmission: This transmission requires a fluid meeting the DEXRON HP (High Performance) specification. This is a full synthetic, low-viscosity fluid designed specifically for the higher stresses and tighter tolerances of GM's 8- and 10-speed transmissions. Using regular DEXRON VI in an 8L90 can lead to shift problems and accelerated wear.

Always check your owner's manual or the transmission fluid dipstick (if equipped) to confirm which fluid your truck requires. Using the wrong fluid can cause serious damage.

Service procedure

Properly servicing your Sierra's transmission is key to a long service life. You have two primary methods for changing the fluid:

• Drain and Fill: This is the most common method. It involves removing the transmission pan, draining the 6-8 quarts of fluid contained within it, replacing the transmission filter, cleaning the pan and magnet, and reinstalling the pan with a new gasket before refilling with fresh fluid. This process only replaces about half of the total fluid in the transmission, as a significant amount remains in the torque converter and cooler lines. It is the GM-recommended procedure.

• Full Fluid Exchange: This process uses a specialized machine that connects to the transmission cooler lines. The machine pumps all of the old fluid out while simultaneously refilling the transmission with new fluid, replacing nearly 100% of the old ATF. While more thorough, this service is also more expensive and must be performed carefully to avoid introducing air into the system or over/under-filling. GM has issued service bulletins cautioning against using flush machines that use harsh cleaning chemicals.

For either service, checking the fluid level correctly is critical. These transmissions do not have a traditional dipstick. The level must be checked via a fill plug on the side of the transmission case while the engine is running and the transmission fluid is at operating temperature (typically 160-180°F).

Service interval

General Motors provides two service intervals for the 2018 Sierra 1500 based on usage:

• Normal Service: For trucks that see mostly highway driving and light-duty use, GM recommends a transmission fluid and filter change every 90,000 miles.

• Severe Service: This is the more common interval for truck owners. If you frequently tow trailers, haul heavy loads, use your truck for commercial purposes, drive in heavy city traffic, or operate in mountainous terrain, you should follow the severe service schedule. This calls for a fluid and filter change every 45,000 miles.

Following the severe service schedule is a wise investment in protecting your transmission from premature failure, especially when towing or hauling.

Common transmission problems

While generally robust, both the 6L80 and 8L90 transmissions have some known issues to be aware of:

• 6L80: Some owners have reported hard shifts (particularly the 1-2 shift), torque converter shudder, or a delay when shifting into drive or reverse. These issues can sometimes be traced to a faulty torque converter, worn clutch packs, or valve body issues.

• 8L90: The most widely reported issue with the 8L90 is a "shudder" or vibration that occurs under light to moderate acceleration, often feeling like driving over a rumble strip. This has been traced to the original factory-fill DEXRON HP fluid absorbing moisture, which degrades its performance. GM has issued a technical service bulletin (TSB 18-NA-355) that involves flushing the original fluid and replacing it with a newer formulation of Mobil 1 Synthetic LV ATF HP fluid. Hard shifting or hesitation are also reported by some owners.

Keeping up with your fluid and filter changes using the correct, high-quality fluid is the best way to prevent these issues and ensure your transmission shifts smoothly for years to come.

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