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API Basestock Classification

October 01, 2012
Jean Van Rensselar
Tip or Best Practice

API Basestock Classification

The American Petroleum Institute (API) categorizes basestocks by their sulfur content, level of saturates and viscosity index (VI). There are five API basestocks classification:

  • Group I Solvent Dewaxed Basestock is the least refined – usually a mix of different hydrocarbon chains. These oils are generally used in application without high performance demand but have significant advantages when it comes to additives
  • Group II Hydroprocessing and Refining Basestock is common in commercial mineral-based motor oils. They perform acceptably in terms of volatility, oxidative stability and flash/fire points but not as well in terms of cold-start viscosity, extreme pressure durability and pour point. Greater purity means that the basestock and the additives in the finished product will last much longer.
  • Group III Hydroprocessing and refining Basestock is the most refined of all mineral oil basestock. It performs well in many regards and is highly stable. Lubricants formulated with this basestock are often synthetic or semisynthetic. Group III basestock is manufactured using the same hydroprocessing techniques as Group II basestock. The difference is that the processes are stepped up in order to yield a higher VI. Today’s Group III basestock performs as well or better than traditional synthetic oils.
  • Group IV or PAO (Polyalphaolefin) Basestock is a chemically engineered synthetic. It has a highly stable chemical composition and is increasingly present in synthetic and synthetic-blend products for industrial and vehicle applications.
  • Group V Basestock is usually blended with other basestock. It is used in small amounts as secondary basestock to impact discrete lubricant properties. This basestock is capable of accommodating a wide variety of properties and custom packages. Because of this it provides the base ingredient for many additives.
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