Definition - What does Viscosity mean?
Viscosity refers to the thickness of a fluid. In scientific terms, Viscosity can be defined as the amount of resistance exerted by a fluid until it gradually deforms due to shear stress. It is one of the most important properties of a fluid. Viscosity in oil is expressed in API gravity, e.g., viscous crude is heavy thick crude with an API gravity of less than 10 degrees and flows at a very low speed in a pipeline or a wellbore and needs special equipment to handle.
Petropedia explains Viscosity
The Viscosity of a fluid occurs when molecules of the fluid collide between neighboring molecules moving at different velocities. Viscosity plays a very important role in deciding the price of crude oil as it is expressed in terms of API gravity which is standardized by API. The higher the API gravity of crude oil the lower the viscosity and the higher the market price. Low viscous oil is light and moves swiftly from reservoir to the surface and then into pipelines. Highly viscous oil is heavy and demands more costly processes in its transportation and refining.