2015 Peterbilt 579 Fluid Type Specs

As an ASE-certified master technician, I know that using the correct fluid is just as critical as turning the wrench correctly. Modern trucks like the 2015 Peterbilt 579 are complex machines with very specific fluid requirements. Using the wrong type can lead to premature wear, reduced efficiency, and even catastrophic failure. This guide will walk you through every factory-spec fluid for your 2015 Peterbilt 579.

Quick Reference

Fluid	Type/Spec	Capacity
Engine Oil	API CK-4; SAE 15W-40, 10W-30	11.4 gal / 44 L (PACCAR MX-13)
Engine Coolant	TMC RP-329B or RP-338B ELC	14 gal / 53 L (PACCAR MX-13)
Transmission Fluid	API MT-1; SAE 50 / SAE 80W-90	Varies by transmission model
Rear Differential	API GL-5; SAE 75W-90, 80W-140	3.5 - 7.0 gal / 13 - 26 L (varies)
Brake Fluid	N/A (Full Air Brake System)	N/A
Power Steering	Dexron III / Mercon ATF	~2-3 qts / 1.9-2.8 L
Washer Fluid	Commercial All-Season	1 gal / 3.8 L
Diesel Exhaust Fluid (DEF)	ISO 22241	11-27 gal / 42-102 L

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Engine Oil

The heart of your Peterbilt 579 is its engine, most commonly the PACCAR MX-13. Protecting this investment starts with the right engine oil. For 2015 models, the primary specification to look for is API Service Category CK-4. While the older CJ-4 is acceptable, CK-4 oils offer improved shear stability, oxidation resistance, and aeration control, which is vital for modern, hotter-running diesel engines.

• Specification: API CK-4
• Viscosity: SAE 15W-40 is the standard and most common choice for all-around performance and protection. For improved fuel economy and cold-weather performance, SAE 10W-30 is also an excellent option, provided it meets the CK-4 spec.
• Capacity: For the PACCAR MX-13 engine, the total system capacity is approximately 11.4 gallons (44 liters), including the filter. Always verify the level with the dipstick after a change.
• Notes: Always use oil from a reputable brand. Oil change intervals can be as high as 60,000 miles or more, but these are highly dependent on your duty cycle, idle time, and fuel consumption. It is best practice to follow the recommendations of your engine's electronic monitoring system or to perform regular oil analysis to determine the optimal change interval for your specific operation.

Engine Coolant

Engine coolant in a heavy-duty truck does more than just prevent freezing; it manages the immense heat generated by the engine and prevents corrosion. The 2015 Peterbilt 579 uses a specific type of Organic Acid Technology (OAT) coolant, often referred to as an Extended Life Coolant (ELC).

• Specification: TMC RP-329B (Ethylene Glycol based) or RP-338B (Propylene Glycol based). These are typically red or pink in color.
• Capacity: Approximately 14 gallons (53 liters) for a PACCAR MX-13 equipped model. Total capacity can vary based on options like bunk heaters.
• Notes: Crucially, never mix ELC with conventional coolants. Do not use "universal" or "all-makes, all-models" coolants. Mixing different coolant chemistries can cause them to gel, leading to plugged radiators, heater cores, and catastrophic engine overheating. If you need to top off the system, use only a compatible ELC or distilled water in an emergency. The service life of ELC can be up to 600,000 miles or more, but it should be tested annually for freeze point and additive strength.

Transmission Fluid

Your truck could have one of several manual or automated manual transmissions from Eaton Fuller. These are robust units, but they require the correct fluid.

• Specification: For most Eaton Fuller manual and automated transmissions (like the RTLO-series), the correct fluid is a non-EP (Extreme Pressure) mineral gear oil compliant with API MT-1. Many manufacturers label this as "SAE 50 Transmission Fluid."
• Viscosity: SAE 50 is the standard. In some cases, a high-quality SAE 80W-90 gear oil is also acceptable, but the SAE 50 fluid is preferred for optimal shift quality and efficiency.
• Notes: Do not use standard GL-5 gear oil. The sulfur/phosphorus additives in GL-5 can be corrosive to the brass and bronze synchronizers found in these transmissions. Always verify the spec for your exact transmission model, as some automated variants may have different requirements. Fluid change intervals are typically between 250,000 and 500,000 miles depending on the fluid type and duty cycle.

Rear Differential Fluid

The differentials translate power to the wheels and operate under immense load. The correct gear oil is essential for their longevity.

• Specification: API GL-5 is the universal standard for hypoid gears in differentials.
• Viscosity: SAE 75W-90 synthetic gear oil is the preferred fluid for on-highway applications, offering a great balance of protection and efficiency. For severe service or extreme heat, SAE 80W-140 may be specified. Always check your axle manufacturer's recommendation (e.g., Dana, Meritor).
• Capacity: This varies widely based on the specific axle model, but typically ranges from 3.5 to 7.0 gallons (13 to 26 liters) for a tandem axle setup.
• Notes: Unlike light-duty trucks, limited-slip additives are not typically required unless your truck is equipped with a specific traction-control differential that calls for it. Using a high-quality synthetic GL-5 oil will provide the best protection and may allow for extended drain intervals. Check for and clean the magnetic drain plug during each service to monitor for abnormal wear.

Brake Fluid

The 2015 Peterbilt 579 is equipped with a full air brake system. This system does not use hydraulic brake fluid. The braking force is supplied by compressed air.

Power Steering Fluid

Power steering systems in heavy trucks still rely on hydraulic fluid to provide steering assist.

• Specification: A standard Automatic Transmission Fluid (ATF) is commonly used. Dexron III / Mercon compatible fluids are the typical factory fill.
• Notes: Do not use dedicated "power steering fluid" unless it explicitly states that it meets the Dexron III/Mercon specification. The system is relatively low-capacity, so check the fluid level in the reservoir regularly. Dark, burnt-smelling fluid indicates a problem and should be flushed.

Washer Fluid

A clear windshield is a non-negotiable safety item.

• Specification: Any commercial, all-season windshield washer solvent is acceptable. Choose one with de-icing properties for winter use.

Diesel Exhaust Fluid (DEF)

To meet 2010 emissions standards, your truck is equipped with a Selective Catalytic Reduction (SCR) system that requires Diesel Exhaust Fluid (DEF).

• Specification: The fluid must be certified by the American Petroleum Institute (API) and meet ISO 22241 standards. The label should clearly state this.
• Capacity: Tanks vary in size from 11 to 27 gallons (42 to 102 liters).
• Notes: DEF is a solution of 32.5% urea in 67.5% deionized water. It is not a fuel additive. Never put DEF in the diesel tank, and never put diesel in the DEF tank. Contamination of the DEF system can lead to extremely expensive repairs. Use only sealed, clearly labeled DEF containers and dedicated filling equipment.

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Keeping up with fluid services is the most cost-effective way to ensure your 2015 Peterbilt 579 remains reliable and profitable for years to come. When in doubt, always refer to your owner's manual or OEM service information for the final say.

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