API Base Oil Classification Explained by Eurofins TestOil

API Base Oil Classification Explained by Eurofins TestOil

Additives only account for about 10% of most finished lubricants, the rest is base oil—which gives you some idea as to how important base oil actually is! The API (American Petroleum Institute) classifies base oils into five primary groups that are defined by three parameters: the saturate level, sulfur level, and viscosity index.

  • Saturate Level: While saturates occur naturally in crude oil, they increase during the refining process. The higher the level, the greater the resistance to breakdown, oxidation, and loss of viscosity.
  • Sulfur Level: Sulfur also occurs naturally in crude oil. Generally, the lower the sulfur content, the purer the oil and the less susceptible it is to corrosion and oxidation.
  • Viscosity Index: The VI (viscosity index) refers to changes in viscosity relative to temperature. While all oils increase in viscosity when the temperature decreases and decrease in viscosity when the temperatures increases, when the viscosity index is high the oil is less reactive to temperature fluctuations.[1]

The Five API Base Oil Groups

  • Group I: These are less than 90 percent saturates, greater than 0.03 percent sulfur and have a VI of 80 to 120. Group I base oils are solvent-refined and aren’t recommended for high performance applications.  These are the least costly base oils available.
  • Group II: These are greater than 90 percent saturates, less than 0.03 percent sulfur and have a VI of 80 to 120. They are often manufactured by hydrocracking. These oils have better antioxidation properties and a clearer color than Group I oils, but they aren’t recommended for extreme pressure applications that require cold start viscosity and pour point performance. Group II oils are common in off-the-shelf mineral-based motor oils.
  • Group III: These are also greater than 90 percent saturates, less than 0.03 percent sulfur, but they have a VI over 120. These oils are more refined than Group II oils and are severely hydrocracked to achieve greater purity. Lubricants that result from these base oils are labeled synthetic or semi-synthetic.
  • Group IV: These PAOs (polyalphaolefins) are developed through a synthesizing process. They have a much broader temperature range and are great for use in extreme cold conditions and high heat applications. They can be significantly more expensive than the 1st three groups.
  • Group V: This group encompasses all other base oils, including silicone, phosphate ester, PAG (polyalkylene glycol), polyolester, biolubes and more. These base oils are at times mixed with other base oils to enhance properties. For example, esters are common Group V base oils used in different lubricant formulations to improve the properties of the original base oil.

There are also unofficial classifications of Group II+ (VI >110); Group III+ (VI>130); and Group VI ((synthetic oils made of PIOs (poly-internal-olefins)).[2]

How Base Oil Affects Performance

Eurofins TestOil analysts know that a lubricant’s base oil can have a significant impact on performance. Lubricant properties affected by base oil include:

  • Viscometrics
    • SAE viscosity grade
    • Pour point and low temperature fluidity
    • Fuel economy
    • Wear Protection
  • Oxidation
    • Viscosity increase
    • Acid formation (leading to corrosion)
    • Deposit control
  • Dispersancy and solvency
    • Soot Control
    • Viscosity increases and filter plugging
    • Sludge
    • Deposit control
  • Foaming and air entrainment
  • Volatility (evaporation)
    • Oil consumption and Flash Point[3], [4]

Regardless of the lubricant’s properties, a comprehensive oil analysis program is critical to optimizing lubricant performance. Working with a lab, like Eurofins TestOil, that knows the unique properties of base oils and their unique performance characteristics will ensure a more accurate picture of lubricant health.

                For more information on working with Eurofins TestOil for oil analysis and training visit www.testoil.com. Contact: 216-251-2510; sales@testoil.com.

                With more than 30 years of experience in the oil analysis industry, Eurofins TestOil, focuses exclusively on assisting industrial facilities with reducing maintenance costs and avoiding unexpected downtime through oil analysis program implementation. As industry experts in diagnosing oil-related issues in equipment such as turbines, hydraulics, gearboxes, pumps, compressors and diesel generators, Eurofins TestOil provides customers with a guarantee of same-day turnaround on all routine testing. Eurofins TestOil PRO certified lubrication professionals educate the industry on oil analysis through public and private onsite training. These highly experienced professionals provide a variety of onsite services; from collecting oil samples to conducting failure analyses to writing work orders. For more information on partnering with Eurofins TestOil on oil analysis programs or training opportunities visit https://testoil.com. Contact: Michael Barrett 216-251-2510; sales@testoil.com


[1] From https://www.rymax-lubricants.com/updates/what-are-the-differences-in-base-oil-groups/

[2] From Wikipedia entry on Base Oil: https://en.wikipedia.org/wiki/Base_oil

[3] From Infineum “Lubricant Base Oils” May be available at: https://www.infineuminsight.com/media/1814/3-lubricant-base-oils-na.pdf

[4] See also Eurofins TestOil blog post: https://testoil.com/did-you-know/how-base-oil-affects-lubricant-performance/

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