Fluid Shear
The mechanical stress that breaks down viscosity index improvers in motor oil, reducing its lubricating effectiveness.
Fluid shear is the mechanical stress and sliding action experienced by automotive lubricants as they are compressed between fast-moving metal parts. This occurs in high-stress areas of the engine and drivetrain, such as between the piston rings and cylinder walls, within high-pressure oil pumps, and between the meshing teeth of gearbox and differential gears.
Under high shear stress, the long-chain polymer molecules (viscosity index improvers) blended into multi-grade motor oils are physically torn apart. This molecular breakdown reduces the oil's viscosity, causing it to become thinner than intended. As the oil thins out, its film strength decreases, allowing metal-on-metal contact and increased component wear under high temperatures.
Severe fluid shear can be detected by monitoring oil pressure; as the oil degrades, hot idle oil pressure will drop. Regularly scheduled oil changes are essential to replace sheared oil before it loses its lubricating capacity. Mechanics recommend high-shear-stability synthetic oils for heavy towing or high-performance applications where shearing forces are extreme.