Fundamentals of Base Oil: Understanding Physical Characteristics

Published by: Chevron Technology | 15-Feb-2023

Transport is as old as lubrication. The wooden axles of the horse-drawn carts of the past were lubricated with tallow and leftover meat greases. Later, hog fat and pine tar were combined to serve as a lubricant. Over time, linseed oil, which was first created as a wood preservative became the coachmen's preferred lubricant.

The modern day base oil was produced by the refining of crude oil, which was the oil used in early automobile engines. Engine technology developed, necessitating improved lubrication due to complex, fast-moving parts and high temperatures. 

In order to improve corrosion resistance, boost viscosity, and lessen friction and wear, additives were added.

Still, the performance of the final product is primarily determined by its base oil. In modern-day passenger car motor oils, the base oil accounts for about 75% to 80% of the final product. An additional 10% to 20% is made up of the additive package. An additional 5% to 10% goes towards a viscosity index improver, which is used to reduce the amount at which viscosity will fall due to high temperatures. Remaining 1% is made up of multiple inhibitors.

Crude oil can be refined to produce base oil. Although over 45 gallons of petroleum products can be produced from a 42-gallon barrel of crude oil, only around 0.4 gallons, or less than 1%, goes to making lubricants. The majority is used for jet fuels of the kerosene type, diesel fuel, and gasoline.

The American Petroleum Institute divides base oils into five categories, groups labeled I-V, according to how the oils are processed.

Group II oils have improved oxidation stability, low amounts of sulfur, nitrogen, and aromatics, and higher purity as compared to the less refined Group I oils. The deeper hue of finished motor oil is imparted by the additives; pure Group II base oil is as transparent as water. Group I oils are becoming less common as they are not suitable for application requiring premium base oils. Many Group I uses can be replaced with Group II oils.  The base oils in these Groups (I and II) are commonly termed as "mineral conventional base oils."

High-quality base oils classified as Group III and IV are designed to be used in low viscosity, high performance motor oils (such 0W-20) in highly developed automobile engines. Oils made from these base oils are classified as synthetics. They support increased fuel efficiency, have better oxidation characteristics, and might permit longer drain intervals.  Group IV, also referred to as "poly-alpha olefins" or PAOs, is the only base oil considered to be genuinely synthesized.

Depending on the application, lubricant and automotive manufacturers have employed Groups I-V base oils. Selecting the "correct base oil" for the engine oil formulation can help achieve greater performance in demanding applications such as stop-and-go traffic, extreme cold climates, high temperature performance in turbochargers, and long drain intervals.

Following four physical characteristics of base oils determine their performance during  service:

  1. Pour point. The minimum temperature at which a sample of oil can be poured is the pour point.
  2. Viscosity. The viscosity of an oil is defined by its resistance to flow. For instance, honey has a higher viscosity than water.
  3. Viscosity index (VI). An oil's VI is determined by its viscosity, which varies with temperature. For instance, the viscosity of a high-VI oil varies less with temperature than that of a low-VI oil. High-VI base oils are required to begin the formulation process for the multi-grade engine oils that automakers specify. Base oils with a high volatility index (VII) are engineered to function at both elevated and lowered temperatures.
  4. Purity. Sulfur, nitrogen, and polycyclic aromatic compounds—components of many lubricants—must be kept within very strict parameters.

The most important thing to keep in mind about base oils is that they supply a significant portion of the finished oil formulation's performance attributes. Choosing the correct base oil type is important in producing oils that will keep metal parts lubricated and equipment performing at its peak. 

Base oils are just a part of the formulation in oils. Engineers and scientists must also take additive technology's effects into account. Any lubricant's ultimate efficacy is determined by its base oil composition, additives, and application-specific formulation expertise.

This Article is written by Chevron Technologies In collabaration with Industry Expert