Viscosity

• The viscosity grade of the oil, set by the Society of American Engineers, refers to its fluidity or thickness. In the viscosity number of 10W40, for example, the first number refers to how quickly the car engine will start when cold or in winter. The second number refers to the viscosity of the oil when the car has been running for a while.
  
  

Performance

• The performance level of the oil for engines is found on the top part of the American Petroleum Institute (API) symbol on the oil can label. Oil suitable for gasoline engines is marked with an "S" followed by another letter.
  
  

Energy Conservation

• The rating at the bottom of the API symbol identifies the oil as having energy conserving properties. Along with this is a categorization, CI- Plus, which marks motor oils that provide a high level of protection against viscosity increase or viscosity loss.