Determining which engine oil is "better" between 5W-40 and 15W-40 depends primarily on the operating temperatures your vehicle will experience, particularly at startup. Based on viscosity ratings, the key difference lies in their performance in cold conditions.
Engine oil viscosity is measured at different temperatures to determine how well it flows. Multigrade oils, like 5W-40 and 15W-40, are designed to perform across a range of temperatures. The numbers indicate their viscosity:
- The number before the "W" (like 5W or 15W) relates to the oil's flow characteristics at low temperatures. The “W” stands for “winter.”
- The number after the "W" (like 40) indicates the oil's viscosity at high operating temperatures (typically measured at 100°C).
Understanding the Difference
The most significant distinction between 5W-40 and 15W-40, according to the reference, is their behavior in cold weather:
- 5W-40: Has a lower "W" number, indicating better flow at lower temperatures.
- 15W-40: Has a higher "W" number, indicating thicker flow compared to 5W-40 in cold conditions.
As the reference states: "So, for example, a 5W-40 engine oil and a 15W-40 engine oil will have the same high-temperature flow characteristics (40), but the 5W- will flow better and faster at lower temperatures than the 15W-."
This means that during a cold start, the 5W-40 oil will circulate through the engine more quickly than the 15W-40, providing faster lubrication to critical components. Both oils, however, will have the same target viscosity (40) once the engine reaches normal operating temperature.
Practical Implications
Choosing between 5W-40 and 15W-40 should be guided by your vehicle's manufacturer recommendations and the climate you operate in.
- For Cold Climates: If you frequently start your vehicle in freezing or sub-zero temperatures, 5W-40 is generally preferred because its better cold flow offers superior protection during startup, reducing wear.
- For Warmer Climates or Heavy-Duty Use (where recommended): 15W-40 might be suitable, but 5W-40 still offers the advantage of better cold-start protection while providing the same high-temperature viscosity as 15W-40.
Here's a quick comparison based on the low-temperature flow:
| Feature | 5W-40 Oil | 15W-40 Oil |
|---|---|---|
| Low-Temperature Flow | Flows better/faster | Thicker flow |
| High-Temperature Viscosity | Same (40) | Same (40) |
| Cold Start Protection | Generally superior | Less protective than 5W-40 in cold |
| Best Suited For | Colder climates, where recommended | Warmer climates, where recommended |
Ultimately, "better" is relative to the operating conditions and the specific requirements of the engine. However, in terms of providing faster lubrication and reducing wear during cold starts, 5W-40 has an advantage due to its lower cold-start viscosity compared to 15W-40, as highlighted by the meaning of the "W" rating. Always consult your vehicle owner's manual for the recommended oil viscosity.
