Oil companies developed multi-grade oils in the 1940s. These oils, like SAE 10W-30, meet both low- and high-temperature grade specifications. By adding Viscosity Index Improvers (VII), which are large oil-soluble polymers, the oils resist thinning as they heat up.
For instance, a 10W oil thins to about 5 cSt at operating temperature, but with VII, it maintains a viscosity of about 10 cSt, behaving like a 30 grade oil when hot. VII polymers expand at high temperatures to reduce thinning and contract at low temperatures with minimal impact on viscosity. Examples of VII include Olefin Co-polymers (OCP) and Polymethacrylates (PMA).