2025 Peterbilt 579 Fluid Capacity & Type Specifications

As an ASE-certified master technician, I know that keeping a heavy-duty workhorse like the 2025 Peterbilt 579 on the road requires strict adherence to factory fluid specifications. Whether your rig is equipped with the proprietary PACCAR MX-11/MX-13 engine or the legendary Cummins X15, running the wrong oil, coolant, or gear lube can lead to catastrophic downtime and voided warranties.

Below is your comprehensive master tech reference guide for the 2025 Peterbilt 579 fluid types, specifications, and capacities.

Quick reference

Note: Capacities are approximate and vary based on specific engine options, transmission spec, oil pan size, and chassis configuration. Always verify in your owner’s manual or OEM service info before filling.

Fluid	Type/Spec	Capacity
Engine Oil (PACCAR MX-13)	10W-30 or 5W-30 API FA-4 / CK-4	~42 qts (10.5 gal)
Engine Oil (Cummins X15)	10W-30 or 15W-40 API CK-4, CES 20086	~44 qts (11 gal)
Engine Coolant	OAT ELC (Nitrite-Free, Red)	~12 - 16 gallons
Transmission (Eaton Endurant)	Eaton PS-386 Synthetic Transmission Fluid	~16 pints (8 qts)
Transmission (Allison Auto)	TES 668 Synthetic ATF (TranSynd)	~39 qts (varies by pan)
Rear Differentials (Tandem)	75W-90 Synthetic API GL-5, SAE J2360	~24 - 30 pints (per axle)
Steer Axle Hubs	75W-90 Synthetic API GL-5	~2 - 3 pints (per hub)
Transfer Case	N/A (Unless AWD spec - use 75W-90 API GL-5)	Varies
Power Steering	Dexron III ATF or TRP Power Steering Fluid	~4 - 5 quarts
Brake Fluid	N/A - Air Brakes Standard	N/A
Diesel Exhaust Fluid (DEF)	API Certified ISO 22241	15 - 30 gallons (varies)
Windshield Washer Fluid	Standard All-Season / Winter Blend	~1 - 1.5 gallons

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Engine Oil

Specification: API FA-4 (PACCAR preferred for fuel economy) or API CK-4 (Cummins X15 preferred). The 2025 Peterbilt 579 most commonly features either the PACCAR MX-13 or the Cummins X15. For the MX-13, PACCAR heavily pushes 10W-30 or 5W-30 meeting the API FA-4 specification to maximize fuel efficiency. However, API CK-4 is fully backward compatible and widely used. For the Cummins X15, Cummins Engineering Standard (CES) 20086 dictates API CK-4 in either 10W-30 or 15W-40. Common Pitfall: Do not mix FA-4 and CK-4 if you can avoid it. FA-4 has a lower High-Temperature High-Shear (HTHS) viscosity designed specifically for newer engines. Putting FA-4 in older equipment not rated for it can cause accelerated wear, so keep your bulk tanks labeled clearly. Extended drain intervals (up to 75,000 miles) are possible only with an active oil analysis program.

Engine Coolant

Specification: Extended Life Coolant (ELC) Organic Acid Technology (OAT), Nitrite-Free. Color: Red/Magenta. (Meets PACCAR spec or Cummins CES 14603). Heavy-duty diesel engines in the Peterbilt 579 utilize Nitrite-Free OAT ELC. This coolant is formulated to protect wet sleeve liners from cavitation without the need for Supplemental Coolant Additives (SCAs) at every service interval. Common Pitfall: Never mix OAT coolant with older HOAT (Hybrid OAT) or conventional green/purple coolants. Mixing coolant types drastically degrades the corrosion inhibitors and can lead to gel formation, water pump failure, and catastrophic wet sleeve pitting. Always top off with 50/50 pre-mixed Nitrite-Free OAT.

Transmission Fluid

Specification: Eaton PS-386 (for Eaton Endurant/Fuller) or TES 668 (for Allison automatics). Most 2025 Peterbilt 579s rolling off the line feature the Eaton Endurant HD automated manual transmission (AMT). This unit strictly requires Eaton PS-386 synthetic transmission fluid (often branded as Roadranger). If the truck is spec’d with an Allison heavy-duty automatic, you must use a TES 668 approved fluid (like Castrol TranSynd 668), which supersedes the older TES 295 spec. Common Pitfall: Never use universal ATF, Dexron VI, or Mercon LV in a heavy-duty commercial transmission unless explicitly directed by the manufacturer for a specific lightweight model. Using the wrong fluid in an Eaton Endurant will destroy the synchronizers and mechatronic actuators.

Differential / Drive Axle Gear Oil

Specification: 75W-90 Synthetic Gear Lubricant, API GL-5, SAE J2360 (Dana SHAES 256 Rev C or Meritor O-76-N). The tandem rear axles (whether PACCAR, Dana, or Meritor) require a high-quality fully synthetic 75W-90 API GL-5 gear lube. High-temp applications or heavy haulers operating in severe service may jump to a 75W-140 synthetic, but 75W-90 is the factory standard for highway efficiency. Common Pitfall: Do not underfill the axles. Truck differentials require the fluid to flow to the wheel hubs. When changing the fluid, fill the pumpkin, wait a few minutes for the fluid to run down the axle tubes to the hubs, and then top it off again. Alternatively, jack up each side to force the fluid into the hubs.

Steer Axle Hubs (Front Differential/Hubs)

Specification: 75W-90 Synthetic API GL-5. Most Peterbilts use oil-lubricated front wheel bearings. Inspect the clear plastic hub caps regularly. The fluid used here is the same 75W-90 synthetic API GL-5 used in the rear differentials. Keep the level between the "ADD" and "FULL" marks on the clear cap.

Power Steering Fluid

Specification: Dexron III ATF or heavy-duty power steering fluid. The TRP (PACCAR's aftermarket brand) power steering system generally uses a basic Dexron III ATF. Ensure the system is bled properly if the steering gear or lines are replaced, as air trapped in the Sheppard or TRW steering boxes can cause dangerous steering binding and pump whine.

Brake Fluid

Specification: N/A - Air Brakes. The 2025 Peterbilt 579 is a Class 8 truck equipped with a pneumatic (air) braking system (S-cam drum brakes or air disc brakes). There is no hydraulic brake fluid to check. However, if your specific truck has a hydraulic clutch linkage (rare on newer AMTs but possible on manuals), it will use standard DOT 3 or DOT 4 brake fluid. Always keep the air tanks drained of moisture to protect the pneumatic system.

Diesel Exhaust Fluid (DEF)

Specification: ISO 22241 compliant API Certified DEF. DEF is critical for the Selective Catalytic Reduction (SCR) system to reduce NOx emissions. It is a strict mixture of 32.5% high-purity urea and 67.5% deionized water. Common Pitfall: Never add tap water, diesel, or any additives to the DEF tank. Even a microscopic amount of petroleum can permanently poison the SCR catalyst, leading to a multi-thousand-dollar repair and engine derate.

Windshield Washer Fluid

Specification: Standard automotive washer fluid. Use a winter-grade formula (-20°F or lower) in freezing climates to prevent the lines and reservoir from cracking.

Maintaining your Peterbilt 579 with the correct OEM-specified fluids is the only way to ensure maximum uptime and profitability. If you need help servicing your rig or diagnosing a complex emissions system issue, don't trust your livelihood to just anyone. Find a trusted shop on Repairius.