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. The second number (30) indicates how the oil flows when the engine is hot. The first number (5W) refers to how the oil flows when the engine is cold. On a bottle of multigrade engine oil, like Shell Rotella ® T6 Multi-Vehicle, for example, you’ll see two numbers: 5W-30. Each motor oil label will vary in design, but most will contain one or two numbers specifying the oil’s viscosity grade.
