Oil Viscosity
A measure of an engine oil's resistance to flow at specific temperatures, critical for proper lubrication.
Oil viscosity is the physical measurement of an engine lubricant's resistance to flow. It determines how thick or thin the oil is at different temperatures. Viscosity is classified by the Society of Automotive Engineers (SAE) using numbers, such as 5W-30. The first number (followed by 'W' for Winter) indicates the oil's flow rate at cold temperatures, while the second number indicates its thickness at the engine's normal operating temperature (212°F / 100°C).
At cold startup, oil needs to be thin enough to flow quickly to the top of the engine and lubricate critical valve components. As the engine warms up, the oil must remain thick enough to maintain a protective film between moving metal parts, preventing metal-on-metal wear under load. Multi-grade oils use viscosity index improvers (polymers) that expand as they heat up, preventing the oil from thinning out excessively.
Using the wrong oil viscosity can lead to serious engine problems. Oil that is too thick will restrict cold-start lubrication and increase engine drag, reducing fuel economy. Oil that is too thin will fail to provide adequate film strength at high temperatures, leading to accelerated bearing wear and low oil pressure. Always follow the manufacturer's recommended oil viscosity, typically found on the engine oil cap or in the owner's manual.