The "W" in 15W-40 stands for "winter," and it indicates that the oil is suitable for use in cold weather. The "15" in 15W-40 indicates the oil's viscosity at 0°C (32°F). The lower the number, the thinner the oil.
The "40" in 15W-40 indicates the oil's viscosity at 100°C (212°F). The higher the number, the thicker the oil.
So, 15W-40 oil is a 15-weight oil at 0°C (32°F) and a 40-weight oil at 100°C (212°F).
Here is a table that shows the viscosity of 15W-40 oil at different temperatures:
| Temperature | Viscosity |
|---|---|
| 0°C (32°F) | 15W |
| 25°C (77°F) | 20W |
| 50°C (122°F) | 30W |
| 100°C (212°F) | 40W |
As you can see from the table, the viscosity of 15W-40 oil increases as the temperature increases. This is because the oil molecules become more active at higher temperatures, and they move around more freely. As a result, the oil becomes thicker.
The viscosity of oil is important because it affects the oil's ability to lubricate engine parts. If the oil is too thin, it will not be able to provide adequate lubrication, and the engine will wear out prematurely. If the oil is too thick, it will not be able to flow properly through the engine, and the engine will overheat.
15W-40 oil is a good choice for most cars and trucks in moderate climates. It is thin enough to provide adequate lubrication in cold weather, and it is thick enough to provide adequate lubrication in hot weather.
