High viscosity synthetic base stocks (HVSB) are low molecular weight molecules designed to provide shear stable thickening to high performance applications. HVSBs are available as pure liquid low molecular weight polymethacrylates, proprietary polyolefins, and ethylene-propylene oligomers.
What is the difference between a synthetic base stock and a viscosity modifier (or “VI improver”)?
A viscosity modifier or VI improver is used to adjust the kinematic viscosity of a lubricant at 40C and/or 100C and improve the low temperature fluidity of an engine oil, hydraulic fluid, gear oil, etc. versus using a heavy petroleum base oil like 600N or bright stock. Viscosity modifiers are used sparingly since the high molecular weight (size) of the polymer required to greatly affect viscosity index makes viscosity modifiers capable of being torn apart during intense use in an engine or gearbox.
High viscosity base stocks typically meet OEM gear and hydraulic fluid levels of shear stability (<15 SSI by CEC L-45-A-99 aka “20 hour KRL”) and allow shear stable high performance formulations even at very high treat rates. Molecular weights are typically only 10-20% the molecular weight of a VI improver. These specialized molecules provide more durable lubricants and greases for extended life or extreme conditions of operation.