What does the "W" stand for in 15W-40 motor oil?

The "W" in 15W-40 motor oil stands for "Winter." This designation indicates the oil's viscosity rating in cold temperatures, which is crucial for ensuring that the engine can start and operate smoothly in colder weather conditions. The number preceding the "W," which is 15 in this case, represents the oil's viscosity grade when tested at low temperatures. A lower number means better flow in cold weather, which is vital for engine protection and performance during winter months. The second part of the rating, "40," indicates the oil's viscosity at higher temperatures. Therefore, the inclusion of "W" specifically relates to the oil's performance in winter conditions, making it suitable for colder climates where maintaining proper lubrication during startup is essential.