2018 Peterbilt 579 Fluid Type Specs

Quick Reference: 2018 Peterbilt 579 Fluid Specs

Fluid	Type/Spec	Capacity
Engine Oil	15W-40 or 10W-30 API CK-4	~11-12 gal (42-45 L) - Varies by engine
Engine Coolant	ELC (Extended Life Coolant) - OAT	~12-14 gal (45-53 L) - Varies by engine/sleeper
Transmission Fluid	See specific transmission model	Varies significantly
Rear Differential(s)	75W-90 Synthetic Gear Lubricant	~16-22 pints per axle
Brake Fluid	DOT 3 / DOT 4 (for hydraulic clutch)	N/A (Air Brakes)
Power Steering	Power Steering Fluid or ATF	~2-4 quarts
Windshield Washer	-	~1 gallon (3.8 L)
Diesel Exhaust Fluid (DEF)	API Certified / ISO 22241	~20-30 gallons

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Engine Oil

The 2018 Peterbilt 579 was available with several engine options, primarily from PACCAR and Cummins. The correct engine oil is critical for component longevity, emission system performance, and fuel economy.

• Specification: Use a high-quality diesel engine oil meeting API Service Category CK-4. CK-4 oils are specifically formulated for use in high-speed, four-stroke cycle diesel engines designed to meet 2017 and later on-highway greenhouse gas (GHG) emission standards.
• Viscosity Grade: The most commonly recommended viscosity grades are 15W-40 and 10W-30.
  • 15W-40 is the industry standard for most on-highway trucking applications, providing excellent protection in a wide range of operating temperatures.
  • 10W-30 may be recommended for improved fuel economy and better cold-weather starting.
• Capacity: Oil capacity varies significantly based on the specific engine (e.g., PACCAR MX-13, Cummins X15). It typically ranges from 11 to 12 gallons (approximately 42-45 liters). Always verify the exact capacity in your owner's manual or OEM service information and use the dipstick to ensure a proper fill.
• Notes: Always use an oil that meets the engine manufacturer's specific specification, such as Cummins CES 20086 for the X15 engine. Using the wrong oil can lead to premature wear, DPF (Diesel Particulate Filter) plugging, and other costly emission system failures. Change intervals can be as high as 50,000 miles or more, but should be determined by your duty cycle, operating conditions, and oil analysis programs.

Engine Coolant

Proper coolant is essential for managing the immense heat generated by a modern diesel engine. Using the wrong type can lead to corrosion, silicate drop-out, and overheating.

• Specification: The factory-fill coolant for a 2018 Peterbilt 579 is a Nitrited Organic Additive Technology (NOAT) or similar Extended Life Coolant (ELC). These are typically red or pink in color. They are designed for a service life of up to 1,000,000 miles or more with proper maintenance and extender use.
• Capacity: Coolant system capacity can range from 12 to 14 gallons (45-53 liters) depending on the engine and whether the truck is equipped with a sleeper cab and/or auxiliary bunk heater.
• Notes: Crucially, never mix coolant types. Do not add conventional green silicate-based coolants to an ELC system. Doing so will compromise the corrosion protection and can cause a chemical reaction that leads to gelling and plugging of the radiator and heater core. If you need to top off the system, use a pre-diluted 50/50 mix of the same ELC formula or distilled water in an emergency. Always verify the specific ELC type required by your engine manufacturer.

Transmission Fluid

Transmission fluid specifications are highly dependent on the make and model of the transmission installed in the truck (e.g., Eaton Fuller, Allison).

• Specification: This is not a one-size-fits-all fluid. You must identify the transmission model to determine the correct fluid.
  • Eaton Fuller (Manual/Automated): Many modern Eaton transmissions, like the Endurant series, require a PS-386 specification synthetic fluid.
  • Allison (Automatic): Allison automatic transmissions typically require a fluid meeting TES 668 or a compatible previous specification.
• Notes: Using a generic "universal" transmission fluid is a recipe for disaster. These fluids cannot meet the specific frictional and viscosity requirements of modern, heavy-duty transmissions. Always use the fluid specified by the transmission manufacturer to ensure proper shift quality, prevent overheating, and avoid premature clutch and gear wear. Check the data tag on the transmission case for the model number to confirm the correct fluid spec.

Rear Differential Fluid

• Specification: Use a high-quality synthetic gear lubricant meeting API GL-5. The most common viscosity is 75W-90. These synthetic lubricants provide superior protection under the extreme pressures and temperatures found in a Class 8 truck's drive axles.
• Capacity: Capacity is typically 16 to 22 pints per axle. Tandem axle trucks will have both a forward-rear and a rear-rear differential, each requiring a separate fill.
• Notes: Ensure you use a fluid specifically rated for heavy-duty truck axles. While some lubricants are marketed for both automotive and truck use, a dedicated truck axle lubricant is always the best choice. Change intervals are often in the range of 250,000 to 500,000 miles, but refer to OEM recommendations.

Brake and Clutch System

• Brake System: The primary service brakes on a 2018 Peterbilt 579 are operated by compressed air. There is no "brake fluid" for the main system. The air system has an air dryer that must be serviced regularly by replacing the desiccant cartridge to keep moisture and contaminants out of the system.
• Hydraulic Clutch: For trucks equipped with a manual transmission, the clutch actuation system is hydraulic. This small, separate system typically uses standard DOT 3 or DOT 4 brake fluid. Check the cap on the clutch master cylinder reservoir for the correct fluid type. This fluid should be flushed periodically as it absorbs moisture over time.

Power Steering Fluid

• Specification: The power steering system generally uses a fluid specifically labeled as Power Steering Fluid or, in some cases, a specific type of Automatic Transmission Fluid (ATF). It is critical to verify this in the owner's manual.
• Capacity: The system typically holds 2 to 4 quarts.
• Notes: Do not interchange fluid types. Using the wrong fluid can damage seals and compromise steering assist. Look for any specific requirements from the steering gear manufacturer (e.g., TRW, Sheppard).

Windshield Washer Fluid

• Specification: Use any standard commercial windshield washer fluid rated for all-season use, which provides freeze protection in cold climates.
• Capacity: The reservoir typically holds around 1 gallon (3.8 liters).

Diesel Exhaust Fluid (DEF)

DEF is critical for the operation of the Selective Catalytic Reduction (SCR) system, which reduces NOx emissions.

• Specification: Use a high-quality DEF that is certified by the American Petroleum Institute (API) and meets the ISO 22241 standard. The container should clearly state this certification.
• Capacity: DEF tank sizes can vary, but are commonly in the 20 to 30-gallon range.
• Notes: Never put anything other than DEF into the DEF tank. Contamination with diesel fuel, water, or any other substance can destroy the SCR system, leading to extremely expensive repairs. The DEF filler cap is blue to help prevent misfuelling. Use clean dispensing equipment and do not use old or cloudy DEF.

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