15W40

15W-40 is a multigrade engine oil viscosity grade defined by the SAE J300 standard. The first number, 15W, indicates the oil’s flow characteristics at cold temperatures (the winter rating), allowing it to circulate during cold starts. The second number, 40, reflects the oil’s viscosity at operating temperature, indicating its thickness when the engine is hot. Multigrade oils use viscosity modifiers to provide protective lubrication across a wide temperature range.

15W-40 oils are produced in mineral, semi-synthetic, and fully synthetic formulations. They are widely used in both gasoline and diesel engines and are commonly specified for older or high-mileage engines, diesel-powered trucks and equipment, and engines operating under higher heat or load. The exact suitability depends on the engine and manufacturer specifications. API service classifications such as SN (gasoline engines) and CJ-4/CK-4 (diesel engines) may be used with 15W-40 formulations, often alongside ACEA designations such as A3/B4 for European engines. OEM approvals and regional requirements vary, so the correct specification should be confirmed in the vehicle’s owner manual.

Advantages of 15W-40 include robust high-temperature film strength, good wear protection, and wide operating temperature tolerance. Limitations include that some modern, fuel-economy-oriented engines require lower-viscosity oils, and performance in very cold climates can be less favorable than with lighter grades such as 0W or 5W.

Used and maintained appropriately, 15W-40 oils support a broad range of engines, particularly in hot climates, older vehicles, and heavy-duty applications. Regular oil changes and matching the oil specification to the engine are essential.