2018 Chevrolet Silverado 3500HD Complete Fluid Specs

As an ASE-certified master technician, I’ve seen firsthand the toll heavy towing, payload hauling, and daily abuse take on 1-ton trucks. The 2018 Chevrolet Silverado 3500HD is a heavy-duty powerhouse, available with either the reliable 6.0L Vortec gas V8 or the legendary 6.6L Duramax L5P turbo-diesel. Whichever powertrain you rely on, using the exact factory-specified fluids is not just a recommendation—it’s mandatory for longevity.

Whether you’re servicing the Allison transmission, topping off the Duramax cooling system, or performing a routine oil change, guessing on fluid specs can lead to catastrophic failures. Below is the definitive fluid specification guide for the 2018 Chevrolet Silverado 3500HD.

Quick reference

Fluid	Type/Spec	Capacity
Engine Oil (6.0L Gas)	SAE 5W-30 (dexos1 Gen3 / API SP)	6.0 qts (5.7 L) with filter
Engine Oil (6.6L Diesel)	SAE 15W-40 or 5W-40 (API CK-4 / CJ-4)	10.0 qts (9.5 L) with filter
Engine Coolant	DEX-COOL OAT (GM 6277M, Orange)	Gas: 16-17 qts / Diesel: 25-28 qts (verify in manual)
Transmission Fluid	DEXRON-VI / TES 295/668 (Allison)	6.0 - 7.4 qts (pan drop), ~12.5 qts total
Transfer Case (4WD)	DEXRON-VI Automatic Transmission Fluid	~2.0 qts (1.9 L)
Front Differential	SAE 75W-90 Synthetic Axle Lubricant	~2.0 qts (1.9 L)
Rear Differential	SAE 75W-90 Synthetic Axle Lubricant	3.0 - 4.0 qts (verify in manual based on axle)
Brake Fluid	DOT 3 (Delco Supreme 11)	Fill to MAX line
Power Steering Fluid	GM Power Steering Fluid (Part 19329450)	Fill to MAX line on dipstick
Diesel Exhaust Fluid (DEF)	API Certified ISO 22241	~5.3 - 7.0 gallons
Windshield Washer Fluid	GM Optikleen or standard washer fluid	~1.5 gallons

Note: Capacities are approximate. Always verify the final level using the dipstick or fill plug.

Engine Oil

6.0L Vortec V8 (Gas): GM requires 6.0 quarts of SAE 5W-30 engine oil that meets the dexos1 Gen 3 (formerly Gen 2) specification. If dexos is unavailable, a high-quality API SP full synthetic is acceptable. Normal intervals are dictated by the Oil Life Monitor (OLM), but I strongly advise changing it every 5,000 miles if you are towing frequently.

6.6L Duramax L5P (Diesel): This diesel beast takes 10.0 quarts. The factory spec is API CK-4 (or CJ-4). For standard climates, run SAE 15W-40. If you operate in temperatures consistently below 0°F (-18°C), switch to a full-synthetic 5W-40.

Technician Tip: Never skip the filter, and always pre-fill the massive Duramax oil filter to prevent dry-start wear.

Engine Coolant

Both engines use DEX-COOL (GM 6277M spec), an Organic Acid Technology (OAT) coolant that is dyed orange. The capacity varies wildly depending on your engine and heater configurations—the diesel system holds nearly 7 gallons!

Pitfall Warning: Do not mix DEX-COOL with universal "all-makes, all-models" green or yellow coolant, and absolutely never mix it with HOAT (Hybrid OAT) fluids. Mixing coolant types can cause the silicates to drop out, creating a thick sludge that will clog your heater core and block engine water passages. Always mix your concentrated DEX-COOL 50/50 with distilled water, or buy pre-diluted.

Transmission Fluid

6.0L Gas (6L90 Transmission): Uses strictly DEXRON-VI. A standard pan drop and filter change requires about 6 quarts. Do not use universal ATFs; DEXRON-VI is a highly specific, low-viscosity fluid designed for GM's precise clutch clearances.

6.6L Diesel (Allison 1000 Transmission): Factory fill is also DEXRON-VI, but for heavy towing, many heavy-duty transmission specialists and Allison themselves recommend upgrading to a TES 295 or TES 668 approved synthetic fluid (like TranSynd). A pan drop with the spin-on filter replacement takes about 7.4 quarts. The spin-on filter should be replaced every 10,000 miles for maximum transmission life.

Transfer Case (4WD Models)

Unlike older GM trucks that required specialized Auto-Trak II fluid, the heavy-duty transfer cases (Magna/BorgWarner) in the 2018 3500HD run on standard DEXRON-VI transmission fluid. It holds roughly 2 quarts. Drain and fill this every 30,000 to 50,000 miles, depending on how often you engage four-wheel drive and your towing habits.

Differentials (Front & Rear)

Both the independent front suspension differential and the massive rear solid axle require SAE 75W-90 Synthetic Axle Lubricant. For the rear axle (often an 11.5-inch AAM), GM engineered the locking mechanism to function without the need for additional Limited Slip Differential (LSD) friction modifiers. Adding friction modifiers to the factory G80 locker can actually hinder its engagement. Change the gear oil every 50,000 miles under normal use, or 25,000 miles if you tow heavy loads daily.

Brake Fluid

The Silverado 3500HD utilizes a hydroboost braking system. The hydraulic brake lines take DOT 3 brake fluid (GM recommends Delco Supreme 11). Because DOT 3 is highly hygroscopic (it absorbs moisture from the air), it’s crucial to flush the brake system every 2 to 3 years. Moisture in the system lowers the boiling point, which can lead to terrifying brake fade when descending steep grades with a heavy trailer. Do not use DOT 5 silicone fluid. DOT 4 can be used in a pinch as it is miscible, but stick to DOT 3 to keep factory seals happy.

Power Steering Fluid

Because the HD trucks use a hydroboost system (where the power steering pump also provides pressure for the power brakes), fluid health is doubly important. GM recommends their specific Power Steering Fluid (Part 19329450). If your steering groans or the brake pedal feels overly stiff in cold weather, a power steering flush is highly recommended.

Diesel Exhaust Fluid (DEF)

If you have the 6.6L Duramax, you must maintain your DEF system. The fluid must meet the API Certified ISO 22241 standard. The tank holds between 5 to 7 gallons.

Pitfall Warning: DEF degrades in extreme heat and sunlight. Buy DEF from high-turnover retailers to ensure freshness, and never put DEF in the diesel fuel tank, or vice versa. Doing so will result in a repair bill that rivals the cost of a small car.

Windshield Washer Fluid

Top off the reservoir with standard windshield washer fluid (GM Optikleen or equivalent). In freezing climates, be sure to use a fluid rated for -20°F or lower to prevent the reservoir and lines from cracking.

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