2023 International LT Fluid Specs & Capacities

As an ASE-certified master technician who spends a lot of time working on heavy-duty commercial vehicles, I know that keeping a Class 8 truck on the road comes down to meticulous preventative maintenance. The 2023 International LT is a cornerstone of modern fleets, highly regarded for its aerodynamic efficiency and driver comfort. Whether your LT is spec’d with the proprietary International A26 engine or the legendary Cummins X15, getting the fluids right is the difference between a truck that racks up a million miles and one that ends up on the hook.

Below is the complete fluid specification guide for the 2023 International LT, outlining factory-spec fluids, capacities, and common service pitfalls. Because commercial trucks are highly customizable, always verify exact capacities against your build sheet, VIN, or owner’s manual.

Quick reference

Fluid	Type/Spec	Capacity
Engine Oil (A26 12.4L)	API CK-4 10W-30 or 5W-40 (Navistar Spec)	~42 quarts (with filter)
Engine Oil (Cummins X15)	API CK-4 10W-30 or 15W-40 (Cummins CES 20086)	~44 to 48 quarts (with filter)
Engine Coolant	Nitrite-Free OAT ELC (Red/Magenta), Cummins CES 14603 / Navistar	12 - 15+ gallons (varies by sleeper/heater)
Transmission (Eaton AMT)	Eaton PS-386 Synthetic Transmission Fluid	~16 pints (7.5L) (Eaton Endurant)
Transmission (Allison Auto)	Castrol TranSynd or TES-668 / TES-295 equivalent	~30 - 40 quarts (varies by cooler size)
Drive Axles (Differentials)	75W-90 or 80W-140 Synthetic (Dana SHAES 256 / Meritor O-76N)	~30 - 40 pints per axle (varies by axle model)
Brake System	N/A (Air Brakes) - Service Air Dryer Cartridge	N/A
Power Steering	Dexron III / TES-389 equivalent or Synthetic PS Fluid	~4 - 5 quarts
Diesel Exhaust Fluid (DEF)	API Certified ISO 22241	16 to 23+ gallons (varies by tank size)
Washer Fluid	Standard all-season windshield washer fluid	~1 to 1.5 gallons

Note: Capacities are approximate and depend heavily on chassis configuration, sleeper size, dual vs. single drive axles, and specific powertrain options.

Engine Oil

For the 2023 International LT, your engine oil specification relies entirely on which powerplant sits under the hood.

Cummins X15: The Cummins X15 requires oil meeting the Cummins CES 20086 specification, which aligns with API CK-4. Most fleets are moving toward 10W-30 heavy-duty diesel oil for better fuel economy, though 15W-40 is still widely used and perfectly acceptable for extreme high-temperature operations. Expect to use around 11 to 12 gallons (44–48 quarts) during a drain and filter change.

International A26: The 12.4L A26 engine also demands API CK-4 oil. Navistar factory-fill is typically a 10W-30 synthetic-blend or full-synthetic, though 5W-40 is highly recommended for cold-weather operations to ensure proper valvetrain lubrication upon start-up. Capacity is slightly lower at approximately 42 quarts.

Tech Tip: Never use older API CJ-4 or CI-4 oils in a 2023 truck. The advanced aftertreatment systems (DPF/SCR) require the lower ash content of CK-4 to prevent premature particulate filter face-plugging.

Engine Coolant

Modern International LTs use Extended Life Coolant (ELC). Specifically, you need a Nitrite-Free Organic Acid Technology (OAT) coolant, which is usually dyed red or magenta.

• If you have the Cummins, look for Cummins CES 14603 compliance.
• Do not mix this with older HOAT (Hybrid OAT) or conventional green silicated coolants.

Mixing coolants is one of the most common fleet maintenance errors. Doing so causes the corrosion inhibitors to drop out of suspension, leading to abrasive silicates destroying water pump seals and plugging the EGR cooler. If you are unsure what is in the system, test it with ELC test strips or do a complete flush. Total system capacity is massive—often 12 to 15 gallons or more, depending on if you have an auxiliary bunk heater or arctic package.

Transmission Fluid

Transmission fluid specs depend on your selected gearbox:

• Eaton Endurant / Automated Manual Transmissions (AMT): You must use an approved synthetic transmission fluid like Eaton PS-386. This is a specially formulated synthetic lube that provides superior gear protection and lowers operating temperatures compared to older PS-164 specs. Never put standard gear oil or ATF in an Eaton AMT.
• Allison Automatics: If your LT is spec’d with an Allison transmission, use an Allison TES-668 (the newer, backward-compatible standard replacing TES-295) approved fluid, such as Castrol TranSynd.

Drive Axles (Differentials)

Class 8 trucks use robust tandem drive axles, typically sourced from Dana or Meritor. The gold standard for these differentials is full-synthetic gear oil, usually SAE 75W-90 or 80W-140.

• For Dana axles, look for Dana SHAES 256 Rev C.
• For Meritor axles, look for Meritor O-76N.

Using the correct synthetic fluid is critical for maximizing the drain interval (often up to 500,000 miles in linehaul applications). Ensure you clean the magnetic drain plug at every service to monitor for abnormal gear wear.

Brake Fluid (Air Brake System)

The 2023 International LT uses heavy-duty air brakes (either drum or air disc brakes), meaning there is no hydraulic brake fluid (DOT 3/4/5.1) on this chassis.

Instead of checking fluid, your preventative maintenance must focus on the air system. You need to replace the air dryer cartridge (typically a Bendix AD-IS or similar) annually to prevent moisture and oil blow-by from freezing in the lines or damaging the ABS/EBS modulator valves. Drain your wet tank daily!

Power Steering Fluid

The power steering system typically uses a high-quality Dexron III / TES-389 equivalent automatic transmission fluid, or a dedicated synthetic power steering fluid approved by Navistar. Keep the fluid clean and change the power steering reservoir filter element annually. Contaminated fluid will quickly destroy the steering gear seals and the power steering pump.

Diesel Exhaust Fluid (DEF)

The SCR aftertreatment system requires high-quality DEF meeting the ISO 22241 standard (API certified). Because DEF is 32.5% high-purity urea and 67.5% deionized water, it is susceptible to freezing and degradation.

• Never add water to the DEF tank.
• Keep the cap area clean to prevent dirt from entering the tank, which will clog the DEF doser injector and leave your truck derated on the side of the highway.

Windshield Washer Fluid

Use a standard commercial windshield washer fluid. In winter climates, ensure you are using a -20°F or -30°F rated fluid. The large, curved windshield of the LT requires a lot of fluid to clear salt spray, so fleet owners should always carry a spare gallon in the side box.

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Taking care of a 2023 International LT requires stringent adherence to OEM specifications. Heavy-duty fluid specs are not mere suggestions; they are engineered requirements to handle 80,000-pound loads over hundreds of thousands of miles.

If you are looking for reliable preventative maintenance or a shop capable of servicing modern heavy-duty drivetrains, you need professionals who understand these exact specifications. Find a trusted shop on Repairius.