What is viscosity index, and how does it differ for synthetic vs conventional motor oils? The viscosity index is a measure of how viscosity changes with temperature. Because of the irregular shape, size, and weight of conventional base oils, they tend to have a low viscosity index, meaning their flow behavior change a lot with temperature, versus synthetics, which change less. This means synthetic base oils will flow better at cold temperatures (for start up), but remain thicker at higher temper