Viscosity modifiers allow low viscosity fluids to perform like thicker, more lubricious fluids when the lubricant is shear-stressed. Shearing, the process of layers sliding past each other, erodes the effectiveness of low viscosity synthetic lubricants.
Additives can increase the film strength at critical points. They form a supple molecular cushion to enhance the performance, while allowing the bulk phase of the fluid to continue to be low viscosity and efficient for circulation.