What does the 'W' in 5W30 and 10W30 stand for?

The 'W' in designations like 5W30 and 10W30 stands for "Winter." This indicates the oil's viscosity rating at low temperatures. The number preceding the 'W' represents the oil's flow characteristics when cold, with lower numbers indicating better performance in colder conditions. Thus, 5W30 oil will flow more easily in winter temperatures than 10W30 oil, which is important for ensuring adequate lubrication during engine start-up in cold weather. This distinguishes winter oils from the second part of the designation, which indicates the oil's viscosity at higher temperatures. Understanding this classification is crucial for selecting the right motor oil for different climate conditions and ensuring optimal engine performance.