As many people are already well aware, there are two types of base oil typically used in lubricating oils (with the exception of possibly a third being plant based oils, but we will wait for another time to discuss those). Those two are petroleum based oils refined from crude oil and synthetic oils created through human controlled chemical processes. What many people are not aware of is the true differences between these different types of oils and the various subcategories within each type.
In the lubricant industry, base oils are categorized into one of five groups. Those groups are aptly named groups one, two, three, four and five. They are usually designated with the roman numeral of the group number (ie. group I through group V) when written.
Group I –
Group I base oils are the most basically refined oils extracted crude oil. They are derived from a process called solvent refining. Solvent refining is a process where, through multiple steps, different fractions of the crude oil are removed to create more and more pure fractions. Various different solvents may be used during the process at different times in order to remove specific impurities. Solvent refining is the simplest refining process and therefore group I oils are typically the cheapest type of oil on the market.
Group II –
Group II base oils are typically refined using a process called hydrocracking. This process is a more controlled way to refine oils than the solvent refining process used to create group I oils and results in oil molecules that are less prone to oxidation. Group II oils also are required to have a higher viscosity index than group I oils making them more thermally stable with regards to viscosity.
Recently group II oil prices have come down drastically and are competing with group I oils on price. The good news about this is that companies are able to produce greater performance oils without increasing cost and subsequently, price.
Group III –
Group III base oils are further refined using the hyrdrocracking process. The process to create group III oils is referred to as severe hydrocracking because it utilizes higher heat and higher pressure than the hydrocracking process used to create group II oil. It results in even higher purity oil with a viscosity index of 120 or more.
Group III base oils are commonly referred to as “synthetic” and are legally allowed to be marketed as synthetic, but regardless of that fact, they are still refined crude oil. The only true synthetic oils are groups IV and V base oils.
Group IV –
Group IV base oils are 100% synthetic base oils. The chemical name for group IV oils is polyalphaolefin commonly called PAO. PAO oil is typically very pure and extremely stable across wide temperature ranges. PAO resists oxidation and normally remains fluid at very low temperatures.
One drawback to PAO is its poor solubility. This low solubility has two main consequences. First it makes it more difficult to formulate with because not all performance additives are compatible with it. Second, it does not do much to remove and prevent carbon deposits commonly found in engines and therefore, oils utilizing pure PAO base oils rely more heavily on detergency additives that may be depleted throughout the life of the oil.
Another drawback is that PAO is a non-polar molecule which means lubricating films of PAO are generally weaker than the other types of oil requiring more additives to compensate and protect surfaces.
One last disadvantage of PAO base oils is poor compatibility with common rubber seals. PAO tends to shrink many types of rubber and either other types of oil or special additives are required to counteract the shrinking by offsetting it with equal swelling.
Group V –
Group V is basically a catch all for all of the remaining types of base oils. Esters are the most commonly used oils in the group V category but others include silicones, polyalkylene glycols (PAG) and plant based oils which are typically types of naturally occurring esters.
There are many different types of esters and each type has its own strengths and weaknesses. Esters are typically polar molecules which gives them multiple beneficial properties. Most esters exhibit high solubility with other hydrocarbons resulting in a high level of detergency with some types being stronger than others. Esters generally form strong films on metal surfaces resulting in a natural anti-wear property.
A downside to ester base oils is their price. They are typically much more expensive than the other types of base oil and therefore not practical for making lubricants completely using esters. There are some exceptions to this and some specialty lubricants are able to utilize all ester formulations, but these are rare. Most lubricants utilizing esters contain a relatively low percentage of ester compared to the other components.
Another downside to esters is their compatibility with rubber seals. Unlike PAO oils, esters swell commonly used rubber seals. Small concentrations of esters do not typically have a big enough effect on seals, so small concentrations are more common than large concentrations in finished lubes.
Silicones and PAG oils are rarely used to formulate lubricants, especially in publicly sold consumer products. These types of base oil are more commonly used for specialty products in the commercial and industrial lubricant market. One exception to this is DOT 5 brake fluid which is comprised of silicone base fluids.
Silicones and PAG base oils are completely incompatible with hydrocarbon and cannot be mixed. There is a somewhat recent technology of PAG called oil soluble polyalkylene glycols (OSP), but the price and formulating difficulties has made its introduction quite slow if not halt completely.
Plants based oils, or seed oils, seem attractive due to their sharing of many properties with ester base oils. However, most seed oils are highly unstable and quickly oxidize. They often have very poor low temperature viscosity as well giving them a small window of suitable applications. The most commonly known seed oil is probably castor oil extracted from castor beans. It has both good and bad attributes typical of seed oils.
Finally, I would like to end this explanation of base oils with a quick word about comparing different finished lubes based solely on whether they are conventional petroleum oils or synthetic oils. Yes, synthetic oils certainly have various inherent advantageous properties, but that does not automatically make them better. There are many fantastically formulated petroleum based oils on the market and likewise some very low quality synthetic products. Good quality petroleum oil may outperform synthetic oils in many ways. Please don’t get me wrong and think I am bashing synthetics because I am not trying to say that they are bad. All I want to point out is that the base oil alone is not the only aspect of a finished lube to determine its performance and quality.