2024 International LT Transmission Fluid & Service Guide

As an ASE-certified master technician, I regularly field questions from fleet managers and owner-operators regarding transmission maintenance on late-model Class 8 trucks. The 2024 International LT is a flagship over-the-road hauler that sees rigorous daily use. Depending on how your LT was spec’d from the factory, it is most commonly equipped with the Eaton Endurant HD 12-speed automated manual transmission (AMT), though some vocational or specialized builds may feature the Allison 4000 series automatic.

For the purpose of this guide, we will focus on the most popular pairing: the Eaton Endurant HD AMT. Proper fluid management is critical to maximizing uptime and protecting the advanced pneumatics and gearing inside these modern transmissions.

Quick answer

Here are the essential transmission specifications for the 2024 International LT equipped with the Eaton Endurant HD 12-speed AMT:

• Transmission Make/Model: Eaton Endurant HD (12-speed Automated Manual)
• OEM Fluid Spec: Eaton PS-386 (Synthetic Transmission Fluid)
• Drain & Fill Capacity: Approximately 16 pints (8 quarts / 7.5 liters)
• Full Overhaul Capacity: Approximately 16 to 18 pints (Verify with the specific PTO configuration)
• Filter OEM Part Number: Verify with dealer or service manual (lube and pneumatic filters vary by exact chassis spec and build date)
• Recommended Service Interval: 750,000 miles (Linehaul/Normal) / 500,000 miles (Severe Duty)

Note: If your truck is equipped with an Allison 4000 Series, you will need Allison TES 668 fluid. Always consult your VIN-specific build sheet before servicing.

Fluid type explained

The Eaton Endurant HD transmission requires fluid that meets the Eaton PS-386 specification. This is a specialized, fully synthetic, low-viscosity transmission fluid engineered specifically for heavy-duty manual and automated manual transmissions.

PS-386 replaced older PS-164 (synthetic 50W) specs to provide better fuel efficiency and superior cold-weather flow. It contains extreme-pressure additives that protect the gear teeth under high-torque loads while offering excellent oxidative stability to survive the 750,000-mile extended drain intervals. Using an unapproved fluid, such as standard gear oil or conventional ATF, will cause immediate shifting issues, damage the mechatronic actuators, and void your factory warranty. Always look for the "Eaton Approved PS-386" logo on the bucket or drum.

Service procedure

Because the Eaton Endurant HD is an automated manual transmission rather than a traditional torque-converter automatic, the service procedure is a straightforward drain and fill rather than a flush or full exchange.

Drain and Fill Procedure:

1. Preparation: Bring the truck to operating temperature to suspend any contaminants in the oil. Park on a perfectly level surface.
2. Draining: Clean the area around the fill and drain plugs. Remove the fill plug first (to ensure you can refill it), then carefully remove the lower drain plug. Allow the fluid to drain completely.
3. Inspection: The drain plug is magnetic. Inspect it for large metal shavings. A fine paste is normal; sharp metal shards indicate internal gear or bearing damage.
4. Filter replacement: If your maintenance schedule dictates replacing the pneumatic filter for the shift actuator system or the internal lube filter, do so now. (Because part numbers frequently update, always verify with dealer or service manual).
5. Refilling: Install the drain plug and torque to factory specs. Pump the PS-386 fluid into the fill port until it is exactly level with the bottom of the fill hole.
6. Final Checks: Do not overfill. An overfilled AMT will cause churning, overheating, and fluid to push out of the breather.

Service interval

Eaton designed the Endurant HD to be highly reliable with remarkably low maintenance requirements, but intervals heavily depend on how the truck is used.

• Normal Duty (Linehaul): For typical over-the-road, long-haul operations on paved highways, the fluid change interval is a massive 750,000 miles.
• Severe Duty / Towing: If your International LT is running at maximum gross combination weight (GCW) consistently, operating in mountainous terrain, or doing regional/vocational work with frequent stops, the interval drops to 500,000 miles.
• Pneumatic/Air System: Don't forget the air system. The AMT relies on clean, dry air to shift gears. Servicing your truck's air dryer cartridge annually is just as critical to the transmission's health as changing the gear oil.

Common transmission problems

The Eaton Endurant generation is highly refined, but it is not completely immune to issues. In the 2024 International LT, keep an eye out for:

• Air Supply Contamination: The most common cause of AMT shifting failure isn't the transmission itself, but moisture or oil passing through a failed air dryer into the transmission's pneumatic shift actuators, causing sluggish or missed shifts.
• Software Updates: Modern AMTs rely heavily on software to communicate with the Cummins or International engine. Hard shifting or failure to engage a gear is frequently resolved by flashing the transmission control module (TCM) with the latest OEM software.
• Clutch Actuator Wear: While the fluid lasts 750,000 miles, the automated clutch and its release mechanisms may wear out sooner depending on driving habits and route topography.

Maintaining your Class 8 transmission is critical to keeping your freight moving and your business profitable. If you need professional assistance, fluid analysis, or TCM programming, find a trusted shop on Repairius.