Oil 101: What Makes a Base Oil?

Did you know that base oils make up to 90% of the volume of lubricants you use every day? But what is a “base oil” and why is it so important in regard to the conventional vs. synthetic oil debate?

Welcome to Propel Oil 101! We’re continuing our discussion on chemistry and oil by focussing on base oils. If you haven’t already, make sure to go back and check out our previous blog, Oil 101: Introduction To Propel Lubricants, as it will help keep you up to speed with some of the terms and ideas that will be discussed throughout this module. Otherwise, welcome back to Gear Up with Gregg’s!

To begin to understand what a base oil is we first need to look at the different types. Base Oils fall into three major categories: mineral, synthetic, and vegetable.

3 Types of Base Oils

Mineral base oils are the most common and are refined from crude oil. In the refining process, crude oil is manipulated in order to produce products that are less polar, and more saturated with hydrogen, improving the stability of the end product. The primary processes involved in refining crude oil into base oils are solvent extraction/refining, meaning the removal of polar or “bad” molecules, and hydrocracking. Hydrocracking breaks down chemical bonds and adds hydrogen, therefore increasing hydrogen saturation, which increases the oil’s stability. Paraffinic oils, in particular, are the most common type of mineral base oils, but some naphthenic base oils are used throughout the lubricant industry, as they provide better low-temperature characteristics. However, they are more susceptible to oxidation and thus require additives like antioxidants.

Propel Lubricants Oil 101: What Makes a Base Oil Blog

Synthetic base oils are man-made through a series of chemical reactions that synthesize highly stable bases. This series of reactions is called the Fischer-Tropsch process. Essentially during this process, the oil ends up reduced to its basic molecules, then impurities are extracted. That way individual molecules can be customized in a lab to work with today’s sophisticated vehicles and machinery. This type of base oil is essentially a collection of “good” molecules that are built from smaller ones, which results in a higher quality base oil.

Finally, Vegetable base oils are derived from plant oil and are not very common, as they tend to break down significantly faster than mineral or synthetic base oils. On the positive side, vegetable oils can have excellent lubrication properties, far superior to that of mineral oil, and they tend to be biodegradable.

These three categories are not descriptive enough to tell us about the type of base oil that is in our lubricants. So, API, or the American Petroleum Institute, introduced a system that categorized all base oils into five key groups based on their level of Saturates, Sulfur, and Viscosity Index. This classification system certifies base oils and defines guidelines for base oil interchangeability. It also is used worldwide in order to define the building blocks of quality lubricants. Let’s take a look at some of the different API Base oil groups.

API Base Oil Groups

Group I base stocks, sometimes referred to as bright stock, are mineral oils that are largely produced by solvent extraction alone, and finished lubricants which often do not contain any additives. Common applications include large marine engine oils and large industrial engine applications.

Group II and III oils are similar but are further refined by the hydrocracking process. Other methods are utilized in the refinery process too, but these are the major ones we will cover for now. Many lubricants in the Propel line utilize group 2 and 3 base oils.

Group IV base stocks are Poly-Alpha-Olefins or PAOs and are synthetic hydrocarbon compounds that have been used in automotive engine oils since the 1970s. Some still refer to them as the gold standard when it comes to synthetics, as they have a much broader temperature range and are great for use in extreme cold and high heat applications.

Finally, Group V base oils are the catch-all for anything that doesn’t fit into the previous four categories. Many synthetic base oils that are not PAOs are found in this category. They include diesters, polyol esters, polyalkylene (PAG), phosphate esters, and silicones. Each of these base oils provides advantages and disadvantages which is beyond the scope of this lesson.

As this chart indicates, base oils are largely defined by three main characteristics: Viscosity Index, Saturates, and Sulfur.

The higher the number of Saturates, means the higher the molecular bond strength of the oil, and therefore the better the resistance to breakdown or loss of viscosity.
The lower the Sulphur content, the lower the environmental effects and the lesser potential for corrosive damage on modern exhaust systems.
Finally, the higher the Viscosity Index, the less the base oil’s viscosity will change with fluctuations in temperature.
Therefore, less “Viscosity Index Improver additives” will be required, the higher the shear stability will be, and the longer the base oil will last.

Synthetic Oils

Any discussion of base oils lends itself to the discussion of synthetic vs. conventional oils.

Today, base oils from Groups 3, 4, and 5 are considered “synthetic” due to the extent of refining they go through. However, some still consider that only base oils from Groups 4 and 5 are truly 100% synthetic. This is due to these base oils often containing novel compounds that are created by the refining process. By convention, PAO’s or group 4 base stocks, offer higher viscosity indexes, better oxidation stability, and lower volatility. However, the strengths of PAO’s are constrained by their reduced additive solubility, lubricity, seal shrinkage, and reduced film strength. It should be noted that by API, there is no certification to be considered a 100% synthetic oil.

The situation gets even murkier with synthetic blends, as there are no industry standards for how much “synthetic” is actually in a “synthetic blend”. There are many quality synthetic blend products in the market, but you should always look at your product data sheets to really evaluate these products. The overall takeaway should be that highly refined mineral base stocks offer performances very similar to PAO’s. That being said, any decisions about the quality of finished lubricants should always involve a dive into the product’s spec sheets. The proof is in the pudding and spec sheets from reputable blenders will not lie.

Base oils are the foundations upon which lubricants are built. Understanding what category of base oil is in your lubricants can help you to determine not only its strengths but also some of its weaknesses. Next time on Propel Oil 101, we are going to talk about the elephant in the room; oil and the environment. But if you liked what you’ve learned so far, why not subscribe to our YouTube channel? If you have any further questions feel free to contact us, and don’t forget to subscribe to our email list so you can stay up to date with events happening here! Make sure to check out our next Propel Oil 101 blog, right here on Gear Up with Gregg’s Blog!