Oil viscosity is a physical feature that represents the internal resistance of a fluid to flowing. A common mistake is to exchange the concept of viscosity with that of density, which indicates the ratio between the mass and the volume of a liquid instead. Speaking, therefore, of engine oil density to refer to its flowing characteristics is incorrect. An engine oil is said to be viscous if the viscosity is high, and fluid if the viscosity is low. The viscosity of an engine oil is not an unchanging datum, it is influenced by temperature, speed of movement, and pressure.

Oil viscosity index (VI - Viscosity Index) is a number that allows to evaluate the change in the viscosity of a lubricating oil as the temperature changes. It is calculated using the ISO 2909: 2002 method which evaluates the kinematic viscosity of oils at 40°C and 100°C. The higher this value, the greater the stability of the oil will be in terms of viscosity variation. It is around 100 for single-grade oils and over 140 for multigrade oils.

The term single-grade oils, e.g. SAE 10W, SAE 30, SAE 50, indicates engine oils that guarantee either cold or hot performance. On the other hand, multigrade oils guarantee performance both at low and high temperatures e.g. SAE 5W-30, SAE 5W-40. In the case of multigrade oils, the first number followed by the “W” for Winter, indicates its performance at low temperatures, therefore in the start-up phase, while the second number refers to high temperatures and, therefore, refers to the operating phase. Multigrade oils are predominant on the market, and they are obtained by adding additives called VII (Viscosity Index Improvers) to the formula.

SAE International, which stands for Society of Automotive Engineers, is a standardization entity in the aerospace, automotive, and vehicle industries. SAE classifies engine oils based on viscosity only in the SAE J300 table (2015 update):

To simplify, we can say that the table above classifies the different engine oil grades by carrying out some cold measurement tests to evaluate the ignition phase, and some hot tests, to evaluate the operating phase in service. 

• Cold test evaluation: Dynamic cold viscosity (cP) with the Cold Cranking Simulator Test and cold pumpability (cP) with the Mini-Rotary Viscosimeter Test; 
• Hot test evaluation: Kinematic viscosity (cSt) and hot dynamic viscosity at high shear stress (HT-HS High Temperature High Shear Test). 

The correct oil viscosity is always the one indicated by the car Manufacturer’s operation and maintenance manual. It is not recommended to use a different viscosity grade. However, it is common practice, in case of a sporty driving style, to choose a slightly more viscous viscosity because it offers more protection.