High stresses in engine areas such as the oil pump, camshaft, and piston rings can severely affect an oil’s viscosity, causing it to thin out and lose its load-carrying ability under high temperatures and shear forces, leading to engine wear. As shear rate increases, oil viscosity decreases.
Multi-grade oils contain VII, which are large, flexible polymer molecules that expand when heated, entangling with other polymers and trapping oil molecules to maintain viscosity.