Viscosity describes a fluid's resistance to flow. The thicker the fluid, the slower it flows and the higher its viscosity. The viscosity of a lubricant such as oil affects how it reduces friction and transfers heat.
Viscosity in motor oils varies inversely with temperature. In the past, oil was only available in a single grade or weight and had to be changed twice a year for summer and winter. As of 2015, polymers added to oil enable it to maintain a stable viscosity at different temperatures. These multi-grade oils are measured with numbers such as 5w-30. A number followed by W corresponds to winter viscosity, whereas a number sans W corresponds to summer viscosity.
Actual viscosity, not simply the viscosity grade, changes with temperature. Heat lowers viscosity, and coldness increases it. The degree of temperature-related change in viscosity varies by oil. The viscosity index measures the degree of viscosity change relative to temperature. Oils that change viscosity more with temperature have a lower viscosity index than oils that change less with temperature.
The viscosity of mono-grade oils changes more with temperature, so these oils have a lower viscosity index than multi-grade oils. Synthetic oils have higher viscosity indices than mineral oils, giving them a more stable viscosity.