Oil viscosity refers to how thick, or heavy, oil is. The oil viscosity is often used to describe different types of oil customers can buy when getting an oil change in their cars. High-viscosity oil is denser than water; when oil transporters spill oil in water as happened in April 2010, it can cause environmental problems because it does not dissolve in the water.
Motor oil viscosity is a measure of how resistant the oil is to flow, according to the article "Motor Oil Viscosity Explained in Layman's Terms" on Amsoil's website. Higher viscosity oils are thicker and flow more slowly through a viscometer than lower viscosity oils.
Some motor oils are tested for viscosity when they are cold. These oils' viscosity is marked with a W. According to Amsoil, oils that are low viscosity when cold are desirable because the car's engine needs immediate access to oil while still cold. Low viscosity oils will lubricate the engine when it is first started.
Amsoil says that low viscosity oils not marked W are too thin and will cause more wear and tear on the engine. Many oils become thinner as they heat; an oil that is the proper viscosity when cold may be too thin to use when the engine warms up.
Additives can increase oil viscosity so that it does not thin out as it heats up. Amsoil says that additives are added to thinner oils to raise the viscosity at lower temperatures. Then, as the oil heats, it thins to the proper viscosity instead of thinning so much that it hurts the engine. However, as the oil thins, the additive breaks down; eventually the oil will be thinner than usual at high temperatures instead of thicker.
High viscosity oils are heavier than water. If an oil tanker spills oil in the ocean, the oil tends to sink to the ocean floor. This makes clean-up more difficult as well as interfering with fish's ability to receive oxygen from contaminated water.
