International LT Engine Oil
The International LT engine oil spec is API CK-4 meeting Navistar MS-1224 —
15W-40 primary grade, 42 qt capacity (with filter).
FA-4 is not approved for the LT Series A26 engine platform.
FA-4 NOT APPROVED. International/Navistar specifies CK-4 only for the LT (A26) engine. Using FA-4 risks warranty coverage and engine protection.
Quick Spec: International LT Engine
| Engine | International LT (A26 platform) — Inline-6 |
| Displacement | 12.4L (A26) |
| Used In | International LT625, LT625 SBA |
| OEM Oil Spec | API CK-4 / Navistar MS-1224 |
| Primary Grade | 15W-40 CK-4 |
| Cold-Weather Grade | 10W-30 CK-4 (above −10°F / −23°C) |
| Capacity (w/ filter) | 42 qt (10.5 gal) |
| FA-4 Approved? | ❌ NOT APPROVED |
The A26 platform powers both the International LT Series and was used in previous International applications. For the deeper engine teardown and spec history, see our International A26 engine oil page.
Navistar MS-1224 Explained
MS-1224 is Navistar’s proprietary engine oil specification for International heavy-duty diesel engines, including the LT Series A26. It builds on top of API CK-4 — any oil bearing MS-1224 approval must already pass CK-4 performance limits, then clear additional Navistar-specific bench and field tests covering deposit control, wear protection, and oil stability over long drain intervals.
Not every CK-4 oil on the shelf carries MS-1224 approval. The spec isn’t interchangeable with generic CK-4 claims. Navistar publishes an approved oil list, and reputable brands submit their formulations for testing. AMSOIL’s full synthetic CK-4 products, including the Signature Series Max-Duty 15W-40, are formulated to meet MS-1224 performance criteria.
Why does it matter for the LT engine? The A26 is designed around long drain intervals — up to 25,000 miles OEM. Oils that only meet base CK-4 may not maintain adequate protection through those extended periods. MS-1224-capable oils are tested to hold up. When in doubt, verify approval on Navistar’s current approved products list or contact the oil manufacturer directly.
Viscosity Grade: LT Series
International LT operators running the A26 have two approved viscosity options depending on operating temperature. There is no approved FA-4 grade — full stop.
OEM Drain Intervals
The International LT is built for long highway miles. OEM intervals reflect that, but operating conditions matter. Here’s how to read the numbers:
Extended drain intervals require oil analysis, not just a calendar. Pull a sample at 25K, then every 10–15K miles after. Your results tell you when to change — not a generic number. See our OEM drain intervals by engine reference page for context across brands.
AMSOIL Products for the International LT
Two AMSOIL options fit the International LT spec. Both are full synthetic, CK-4 rated, and formulated to meet MS-1224 performance criteria. Pick based on drain interval goals and budget.
- API CK-4 / MS-1224 capable
- Up to 60,000-mile drain w/ oil analysis
- 6x better wear protection vs. conventional
- Maximum shear stability & TBN retention
- Full synthetic formula
- API CK-4 rated
- Meets Allison TES-295
- Full synthetic protection
- Suited to OEM drain intervals
- Strong oxidation resistance
Both products are sold through vyscocity.com, an authorized AMSOIL retailer. Affiliate links support this site at no cost to you.
FAQ — International LT Engine Oil
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