Complete Fluid Specifications for the 2018 GMC Sierra 2500HD

As an ASE-certified master technician, I know that keeping a heavy-duty truck like the 2018 GMC Sierra 2500HD running at peak performance requires strict adherence to factory fluid specifications. Whether your truck is equipped with the proven 6.0L Vortec gas engine or the monstrous 6.6L Duramax L5P turbo-diesel, using the right fluid—and avoiding cheap, "universal" substitutes—is the key to longevity.

Below is the definitive, factory-spec fluid guide for the 2018 GMC Sierra 2500HD.

Quick reference

Fluid	Type/Spec	Capacity
Engine Oil (6.0L V8 Gas)	5W-30 dexos1 Gen 3 (API SP)	6.0 qts (5.7 L) with filter
Engine Oil (6.6L V8 Diesel)	15W-40 (or 5W-40) API CK-4 / dexosD	10.0 qts (9.5 L) with filter
Engine Coolant	DEX-COOL (OAT, Orange) GM 6277M	16.4 qts (Gas) / 25.4 qts (Diesel) - Verify in manual
Transmission Fluid (6L90 - Gas)	DEXRON-VI Automatic Transmission Fluid	6.3 qts (Pan drop)
Transmission Fluid (Allison - Diesel)	DEXRON-VI or Allison TES-668 / TranSynd	7.4 qts (Pan drop)
Transfer Case (4WD)	DEXRON-VI Automatic Transmission Fluid	1.6 qts (1.5 L)
Front Differential (4WD)	SAE 75W-90 Synthetic Axle Lubricant	1.5 qts (1.4 L)
Rear Differential	SAE 75W-90 Synthetic Axle Lubricant	2.7 - 3.0 qts (2.6 - 2.8 L)
Brake Fluid	DOT 3 Hydraulic Brake Fluid	Fill to MAX line
Power Steering Fluid	GM Power Steering Fluid (GM PN 89021185)	Fill to COLD/HOT marks
DEF (Diesel Only)	Diesel Exhaust Fluid (ISO 22241)	~5.3 to 7.0 gallons
Washer Fluid	Optikleen or standard washer solvent	Fill to top

Note: Capacities are approximate. Always verify fluid levels via dipstick or fill plug after servicing, and consult your owner's manual or OEM service info for your specific configuration.

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Engine Oil

6.0L V8 L96 (Gasoline)

For the 6.0L gas engine, GM originally specified dexos1 Gen 2 5W-30, which has now been superseded by dexos1 Gen 3 (API SP). You'll need exactly 6 quarts with a filter change.

• Interval: Follow the Oil Life Monitor (OLM), but never exceed 7,500 miles or 12 months, especially if you regularly tow.
• Pitfall: Avoid older API SN/SM oils, as dexos1 Gen 3 is specifically formulated to prevent Low-Speed Pre-Ignition (LSPI) and timing chain wear.

6.6L V8 L5P Duramax (Diesel)

The Duramax requires heavy-duty diesel oil. The factory fill is generally 15W-40 API CK-4, though 5W-40 API CK-4 synthetic is highly recommended if you live in climates that see freezing temperatures.

• Interval: The OLM will guide you, but severe duty (heavy towing, excessive idling) typically dictates a 5,000-mile interval.
• Pitfall: Do not use legacy CJ-4 oils if you can avoid it, and never use gasoline-rated oil. The high soot and shear loads in the L5P will destroy standard motor oils rapidly.

Engine Coolant

Both engines require DEX-COOL coolant, which is an Organic Acid Technology (OAT) coolant, typically orange or pink in color (GM Spec 6277M).

• Interval: Replace every 5 years or 150,000 miles, whichever comes first.
• Pitfall: Never mix OAT coolant (DEX-COOL) with traditional green Inorganic Acid Technology (IAT) coolant or Hybrid OAT (HOAT) coolants used by Ford or Chrysler. Mixing them will create a sludgy, gel-like substance that will clog your heater core, radiator, and oil cooler. Always use a 50/50 mix of DEX-COOL and distilled water (or pre-mixed coolant).

Transmission Fluid

6L90 (Gas) & Allison 1000 (Diesel)

Both the 6L90 transmission (paired with the gas engine) and the Allison 1000 (paired with the diesel) take DEXRON-VI ATF from the factory. For the Allison, many fleet operators and master techs prefer upgrading to a fully synthetic fluid meeting Allison TES-295 or TES-668 specifications (like Castrol TranSynd), as it offers superior thermal stability for heavy hauling.

• Interval: 45,000 miles for severe service (towing/hauling) or 97,500 miles under normal conditions. Always change the external spin-on filter on the Allison transmission at this time.
• Pitfall: Never use "universal" ATF. DEXRON-VI has a very specific friction modifier package. Universal fluids can cause torque converter shudder and poor shift quality.

Transfer Case

If your 2500HD is 4WD, the transfer case takes DEXRON-VI ATF.

• Interval: Every 45,000 miles under severe duty.
• Pitfall: Do not use gear oil or older DEXRON-III variants. The internal chain and bearings require the low viscosity of DEXRON-VI.

Front and Rear Differentials

Both the front and rear axles require SAE 75W-90 Synthetic Axle Lubricant (GM Part No. 88900401 or equivalent).

• Interval: Fluid should be changed every 45,000 miles under severe duty (frequent towing, off-roading).
• Pitfall: Most 2500HDs are equipped with the GM G80 automatic locking rear differential. Do not add friction modifiers or limited-slip additives (LSD additive). The G80 is a mechanical locker, not a clutch-based limited slip; adding friction modifier will prevent the locking mechanism from engaging properly.

Brake Fluid

The brake system on the 2018 Sierra 2500HD requires DOT 3 Hydraulic Brake Fluid.

• Interval: Brake fluid is hygroscopic (absorbs moisture). It should be flushed every 2 to 3 years regardless of mileage.
• Pitfall: Do not use DOT 5 (silicone-based) fluid. It will ruin the seals in your ABS module and master cylinder. DOT 4 is technically miscible with DOT 3, but sticking to factory DOT 3 prevents any seal compatibility issues over time.

Power Steering Fluid

Unlike the 1500 half-ton trucks that moved to electric power steering, the heavy-duty 2018 2500HD still utilizes a traditional hydraulic steering system (which often includes hydro-boost for the brakes). Use GM Power Steering Fluid (GM Part No. 89021185).

• Interval: Inspect every 7,500 miles. A flush every 50,000 miles keeps the hydro-boost system operating smoothly.
• Pitfall: Do not substitute ATF in this specific system unless authorized by an emergency, as it can cause pump whine and seal degradation over time.

Diesel Exhaust Fluid (DEF)

For the 6.6L Duramax, you must use Diesel Exhaust Fluid meeting the ISO 22241 standard.

• Interval: Top off as needed. The Driver Information Center will warn you well before you run out.
• Pitfall: DEF has a shelf life. Do not buy DEF that has been sitting in a hot gas station window for two years. If DEF goes bad, it will trigger an emissions system fault and potentially put the truck into "limp mode." Also, never put DEF in the diesel fuel tank or vice-versa!

Washer Fluid

Use any standard windshield washer fluid. In cold climates, ensure the fluid has a freeze-protection rating well below your lowest expected temperature to prevent cracking the reservoir or damaging the pump.

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Proper fluid maintenance is the cheapest insurance for your 3/4-ton truck. If you don't have the time, tools, or space to handle these services yourself, don't leave your heavy-duty rig to a quick-lube shop. Find a trusted shop on Repairius.