Base oil basics: quality starts at the base
Lubrication is as old as transportation. The horse-drawn wagons of olden times used leftover meat greases and tallow to lubricate wooden axles. Later, pine tar and hog fat were mixed together for use as a lubricant. Eventually, linseed oil, originally developed as a wood preservative, became the lubricant of choice for coachmen.
Early automotive engines used an oil derived through the refining of crude oil, and the modern base oil was born. As engine technology advanced, intricate, fast-moving parts and high temperatures called for better lubrication. Additives were introduced to reduce friction and wear, increase viscosity and improve resistance to corrosion.
Still, the base oil is the fundamental contributor to the finished product’s performance. In today’s passenger car motor oils, the base oil makes up 75% to 80% of the finished product. The additive package makes up another 10% to 20%. A viscosity index improver, which is added to reduce the degree to which viscosity will decrease due to high temperatures, takes up another 5% to 10%. Various inhibitors make up the remaining less than 1%.
Base oil is produced through the refining of crude oil. A 42-gallon barrel of crude oil can actually yield nearly 45 gallons of petroleum products, but only about .4 gallons or less than 1% goes to making lubricants. The bulk goes to gasoline, diesel fuel and kerosene-type jet fuels.
Base oils are classified by the American Petroleum Institute into five groups labeled I-V based on how the oils are processed.
Group II oils are distinguished from less refined Group I by their higher purity, low levels of sulfur, nitrogen and aromatics, and superior oxidation stability. Pure Group II base oil is actually clear as water – it’s the additives that give finished motor oil its darker color. Group I oils are not suitable for applications requiring premium base oils, and their use is steadily declining. Group II oils can be substituted for many Group I applications. The base oils in these Groups (I and II) are typically referred to as “mineral conventional base oils.”
Group III and IV base oils are high quality oils intended for use in high performance, low viscosity motor oils (such as 0W-20) in technically advanced automotive engines. Oils made from these base oils are classified as synthetics. They exhibit superior oxidation properties, support improved fuel economy, and may allow for extended drain intervals. In some parts of the world, Group IV – also known as “poly-alpha olefins” or PAOs – are considered to be the ONLY base oil that is truly synthetic.
Automotive manufacturers and lubricant producers have used Groups I to V base oils depending on the application. Demanding applications, like high temperature performance in turbochargers, extreme cold temperature climates, long drain intervals, or even stop and go traffic conditions require a higher level of performance that can be achieved by selecting the “correct base oil” for the engine oil formulation.
Base oils are described by four physical properties that dictate how they will perform in service:
- Pour point. The lowest temperature at which a sample of oil can be poured determines the pour point.
- Viscosity. An oil’s resistance to flow defines the viscosity. Honey, for example, is more viscous than water.
- Viscosity index (VI). As an oil’s temperature changes, so does its viscosity, defining its VI. A high-VI oil, for example, changes viscosity less with temperature than a low-VI oil. The multi-grade engine oils specified by vehicle makers require high-VI base oils as a starting place in the formulation process. High-VI base oils have lower volatility and are designed to operate at low as well as high temperatures.
- Purity. Constituents of many lubricants such as sulfur, nitrogen and polycyclic aromatic compounds must be held within strict limits
The key takeaway to remember about base oils is that they provide a large part of the performance characteristics of the finished oil formulation. Selecting the correct base oil type is critical in developing oils that will keep metal parts lubricated and equipment performing at its best. Base oils are only a part of the formulation in oils. Scientists and engineers need to also consider the impact of additive technology as well. The final performance of any lubricant is the combination of base oils, additives, and formulating knowledge for the application.