Both car owners and many workers in the auto service sector have heard of ester oils, but few can correctly explain what they are. Moreover, some firmly believe in the usefulness and indispensability of ester‑based oils, while others consider it just another marketing gimmick. Let's try to figure it out.
In short, esters are complex esters – the products of neutralizing carboxylic acids with alcohols. However, starting the article this “smartly” would be wrong. First, it is necessary to recall the types of base oils used for producing engine and transmission oils.
Base oils, also called the foundations of engine or transmission oil, are produced:
- by crude oil distillation;
- by synthesis from gas or organic acids.
The former are traditionally called mineral, and the latter synthetic base oils.
According to the American Petroleum Institute (API) classification, base oils are divided into five categories:
Group I – base oils obtained by selective cleaning and solvent dewaxing (conventional mineral). Typical characteristics: viscosity index: 80-100; flash point: 190-205°C.
Group II – highly refined base oils with low aromatic and paraffin content, offering increased oxidative stability (hydrotreated oils – improved mineral). Typical parameters: viscosity index: 115-125; flash point – 205-215°C.
Group III – base oils with a high viscosity index, obtained by catalytic hydrocracking (HC technology). Through a special treatment, the molecular structure of the oil is improved, bringing the properties of Group III base oils close to those of the synthetic Group IV base oils. It is no coincidence that oils of this group are classified as semi‑synthetic (and some companies even as synthetic base oils). Typical parameters: viscosity index: 125-160; flash point – 210-225°C.
Group IV – synthetic base oils based on polyalphaolefins (PAO). Polyalphaolefins, produced through a chemical process, have a uniform composition, very high oxidative stability, a high viscosity index and contain no paraffin molecules. Typical parameters: viscosity index: 140; flash point – 250°C.
Group V – other base oils that do not fall into the previous groups. This group includes other synthetic base oils and plant‑based base oils. Typical parameters: viscosity index: 180-200; flash point: 250-330°C.
The chemical composition of mineral bases depends on the quality of the crude oil, the boiling range of the selected oil fractions, as well as the methods and degree of their refining. Mineral base is the cheapest. It is a product of straight‑run crude oil distillation, consisting of molecules of varying length and structure. Because of this heterogeneity, viscosity‑temperature properties are unstable, volatility is high, and oxidative stability is low. Mineral base is the most widely used engine oil worldwide.
1. The feedstock petroleum products distilled in a special vacuum distillation column are divided into distillates and vacuum gas oils.
2. Molecules of wax‑like vacuum gas oils are fed to a hydrocracking unit for hydrogenation.
3. At very high pressure and temperature (300 kPa, 540 °C) the oil molecules become chemically active.
4. Hydrogen is added to the molecules to completely eliminate impurities. Molecular rearrangement occurs, resulting in an exceptional base component for finished lubricants.
5. The transformed component is noticeably lighter in color because it is purer.
6. Undesirable paraffinic hydrocarbons are molecularly restructured, giving the base oil resistance to gelling and excellent flow even at extremely low temperatures.
7. Hydrogen is used again to remove remaining aromatic hydrocarbons and light impurities. It also helps stabilize the molecular structure of the newly formed base component, providing increased oxidative stability and a longer lubricant service life.
Improvement of mineral base oils is carried out in two main directions. The first involves cleaning the oil only to the extent that an optimal content of resins, acids, sulfur compounds, nitrogen remains, and additives are introduced to improve certain functional properties. This method does not yield oils of a sufficiently high quality level. The second direction fully removes all impurities from the base oil and applies molecular modification by hydrocracking. The result is an oil with valuable properties for severe service conditions (high resistance to shear deformation at high speeds, loads and temperatures, high viscosity index and stable parameters).
To which class should such oils be assigned? By price “hydrocracking” is closer to “mineral”, and by quality, as the seller assures, not at all worse than “synthetics”. But we understand that if that were truly the case, such an expensive pleasure as synthetic oil would have become extinct as a class… Hydrocracking oil is closer to mineral not only by price but also by the method of production, because it is also made from crude oil. So what makes it better? As the name suggests, it undergoes deeper processing via hydrocracking. And in the early stages its production does not differ from that of mineral oil. From ordinary mineral oil, various physicochemical methods remove unwanted impurities such as sulfur or nitrogen compounds, asphaltene (bitumen) substances and aromatic polycyclic compounds that increase coking and viscosity dependence on temperature. Depetarization removes paraffins that raise the oil’s pour point. However, it is clear that removing all undesirable impurities by this method is impossible – roughly speaking, this is the cause of the poorest properties of “mineral”. The oil’s processing can continue further. Unsaturated hydrocarbons remain, which accelerate oil aging through oxidation, and impurities also remain. Hydroprocessing (hydrogen exposure at high temperature and pressure) converts unsaturated and aromatic hydrocarbons into saturated ones, increasing the oil’s resistance to oxidation. Thus, oil that has undergone hydroprocessing has an additional advantage.
And hydrocracking is an even deeper type of processing, when several reactions occur simultaneously. Which ones? The same unwanted sulfur and nitrogen compounds are removed, long chains are broken (cracking – literally “breaking”) into shorter ones with a uniform structure, the break points in the newly shortened molecules become saturated with hydrogen (hydrogenation). Hence the name – “hydrocracking”. Thus, during hydrocracking all the signs of synthesis are evident – creation of a new compound from the feedstock, with a new structure and properties. Therefore hydrocracking is often called NS synthesis.
Transport to the Achinsk refinery (Russia) of a hydrocracking reactor. Weight of the unit – 1 300 tonnes plus the railcar intended for its transport – another one‑and‑a‑half thousand tonnes. That’s, overall, the weight of a solid five‑storey building.
But it’s not that simple. Some petroleum components that are usually considered harmful can, in places, be valuable. For example, resins, fatty and naphthenic acids improve the tack and stability of the oil’s adsorption film and thereby enhance its lubricating ability. Some sulfur and nitrogen compounds have antioxidant properties. Thus, during deep oil refining some of its lubricating, antioxidant and anticorrosion properties may deteriorate. This drawback is corrected with special additives that are added at the oil blending plants.
So, hydrocracking oils are products of crude distillation and deep refining. Hydrocracking discards everything “unwanted”, while any “useful” components that are captured are supplied with the necessary properties via additives. However, it is difficult to filter out unwanted impurities completely, so there is more sludge formation and “promotion” of corrosion in hydrocracking oils compared with “synthetic”. Hydrocracking oil approaches “synthetic” quality, but ages faster and loses its properties. Nevertheless, it has a high viscosity index, oxidative resistance and shear deformation stability, and it can often protect against wear even better than a synthetic oil. On the other hand, “synthetic” is more uniform in terms of hydrocarbon chain linearity, which provides advantages such as lower pour point.
There is another aspect. Hydrocracking is a catalytic process, just like synthesis. But while the former may run, for example, on nickel, the latter runs on carbon. Clearly, carbon is preferable in this sense, so the oil will be free of unwanted catalyst compound residues.
The most interesting thing is that the overwhelming majority of motor oils marketed as semi‑synthetic, and even fully synthetic, are nothing more than hydrocracking oils. This is a common trend among the major oil manufacturers. BP’s program (except Visco 7000), Shell (except 0W‑40), most Castrol, Mobil, Esso, Chevron, Fuchs oils are built on hydrocracking. Many very well‑known brands with a full range of oils use only hydrocracking.
Semi‑synthetic is a blend of mineral and synthetic base oils, and can contain from 20 to 40 percent “synthetic”. There are no special requirements for manufacturers of semi‑synthetic lubricants regarding how much synthetic base oil (synthetic component) must be in the finished motor oil – none. There are also no prescriptions on which synthetic component (Group III or Group IV base oil) to use for producing a semi‑synthetic lubricant. In terms of characteristics these oils occupy an intermediate position between mineral and synthetic oils, i.e., their properties are better than ordinary mineral oils but worse than synthetic ones. In price, however, these oils are significantly cheaper than synthetics.
Synthetic oils possess exceptionally favorable viscosity‑temperature characteristics. First, they have a much lower pour point than mineral oils (‑50 °C, ‑60 °C) and a very high viscosity index, which eases engine start‑up in cold weather. Second, they maintain higher viscosity at operating temperatures above 100 °C – thanks to this, the oil film separating friction surfaces does not break down under extreme thermal conditions. Other advantages of synthetic oils include increased resistance to shear deformation (due to the uniform structure), high thermal‑oxidative stability, i.e., low tendency to form deposits and varnishes (varnishes are the clear, very strong, practically insoluble films composed of oxidation products that deposit on hot surfaces), as well as lower evaporation and oil‑consumption compared with mineral oils. Importantly, synthetics require only minimal amounts of thickening additives, and especially high‑grade grades need no such additives at all, making these oils very stable – since additives are the primary source of degradation. All these properties of synthetic oils contribute to reduced overall mechanical losses in the engine and decreased component wear. Moreover, their service life exceeds that of mineral oils by five times or more. The main factor limiting the use of synthetic oils is their high cost. They are 3–5 times more expensive than mineral oils.
All additives are solutions of metals (calcium, zinc, etc.) dissolved in a mineral base oil. Additives are ALWAYS diluted in a mineral base oil, because it combines best with all types of additives. The amount of additives in motor oil varies depending on the oil’s purpose, ranging from 20 to 45 %. Thus, absolutely all motor oils, even “fully synthetic (Fully synthetic)”, are actually blends!
The synthetic base is usually provided by polyalphaolefins (PAO) or esters, or a mixture of them.
PAO are hydrocarbons with a chain length of about 10–12 carbon atoms. They are obtained by polymerizing (simply put – reacting) short hydrocarbon chains – monomers of 3–5 carbon atoms. The feedstock is usually petroleum gases – butylene and ethylene.
Esters are complex esters – the products of neutralizing carboxylic acids with alcohols. The raw material for production is vegetable oils, usually rapeseed or coconut oil. Esters have a number of advantages over all other known bases. First, ester molecules are polar, meaning the electric charge is distributed within them, so the molecule itself “sticks” to metal. Second, the viscosity of esters can be set already at the base‑oil production stage: the heavier the alcohols used, the higher the resulting viscosity. It is possible to dispense with any thickening additives that “burn out” during engine operation and cause oil “aging”. Modern technology allows the creation of fully biodegradable oils based on esters, as esters are environmentally clean products and are easily disposed of. However, all these benefits may seem an overly expensive indulgence. Ester base costs 5–10 times more than mineral base!
So, let’s describe in more detail what the practical use of ester oils actually means.
What do we start the daily operation of a car with? Of course, with engine start‑up. This is the moment many “ailments” appear. For example, a discharged battery, frozen sensors, etc. But these are visible problems. There are also problems hidden from our eyes and senses. The main one is oil starvation during cold engine start. It occurs because, at rest, oil drains into the sump and when the engine starts, the first seconds run without lubrication. Only when the oil is distributed throughout the system does the dry metal‑on‑metal friction cease. Consequently, with each start the engine components suffer significant wear during those friction periods, noticeably shortening the engine’s service life.
Compared with polyalphaolefins, which are simple hydrocarbon chains, ether molecules are polar – the electron density is shifted toward the oxygen atom of the carbonyl group. Hence, the most important advantage of ether‑base oils: the negatively ionized oxygen atom will inevitably be attracted to the metal surface of the lubricated parts, because the crystal lattice of any metal or alloy consists only of positively ionized and neutral atoms.
The cost of an ether base is 5–10 times higher than that of a mineral base, because its production involves several stages. The raw material is usually oil from coconut copra or rapeseed, which is hydrolyzed, separating glycerin and obtaining the required fatty acids.
The final stage is esterification, i.e., the interaction of the acid with an alcohol. Obviously, the culprit is not the molecule with only two carbon atoms, but the heavier ones, which have from 4 to 22 carbons, because the larger the R1 radical in the ether molecule, the higher its viscosity. By the way, this is the second main advantage of esters over polyalphaolefins: the viscosity of the final product can be easily adjusted by using different alcohols.
Additionally, the oil’s properties can be varied by changing the acid radical R, which further increases the cost – then the fatty acids also have to be synthesized. Therefore, plant‑derived oils are used more often, thanks to coconut palms, which are sufficient for motor oils.
With certain combinations of radicals, environmentally clean biodegradable esters are produced: they are already 15 times more expensive than “mineral oil”. An oil made on their basis, when introduced into soil, decomposes by bacteria to 85% within 21 days, although a 66% degradation is sufficient to obtain an ecological certificate.
The second important property is the temperature stability of ester oils, which allows engine protection across all temperature ranges. But what does viscosity depend on in any other oil?
To maintain viscosity, motor oil additive packages contain special thickeners. They are spiral‑shaped molecules that operate on the principle of a helical spring. When the oil is exposed to high temperatures, the spiral expands, but only to a certain extent, keeping the oil’s viscosity within acceptable limits. At low temperatures, the thickener molecules prevent the oil from thickening excessively, acting in the opposite direction. This technology works perfectly until the thickeners are compromised by mechanical stresses. After that, the viscosity stability of the oil depends solely on its base. It should also be remembered that the more additives in the oil, the more sludge formation, which is always detrimental to the engine.
The highest viscosity index of ester oils is directly related to the alcoholic component of the esters – its density influences the viscosity of the final product. Thus, by using more or less dense alcohols in the production of the ester base, developers, as we have already mentioned, initially set the oil’s viscosity parameters. Unreliable thickeners are no longer needed. This means that ester‑base oils do not depend on the presence of thickeners, and their viscosity will remain stable from the start to the end of service.
The ester base also has high flash point values, which sharply reduces oil consumption due to carbon deposits. Its high‑temperature oil film shear values significantly exceed those of any traditional oils, including those formulated with PAO synthetic bases.
Another of the most important requirements when operating units that require lubrication is the strength of the oil film. The protection of friction pairs from wear depends on how strong it is. For this, we present the maximum load that oil films can withstand (under vertical impacts):
- mineral base – 900 kg/cm²;
- synthetic (PAO) – 6500 kg/cm²;
- synthetic (esters) – 22000 kg/cm².
It is clearly visible that the oil film of the ester base is roughly three times stronger compared to the synthetic PAO base. That is why ester‑based oils are so popular in professional auto and motorsport – they are ideal under peak engine loads!
And, in addition to everything said, ester oils have shown the best resistance to oxidation, which is inevitable when using low‑quality fuel (i.e., practically any fuel from Ukrainian gas stations).
In summary, ester oils truly differ greatly from their “counterparts”. Let’s briefly list their main properties and positive “outcomes”:
1. Metal‑adhesion effect – safe engine start.
2. Constant oil viscosity – consistent oil pressure and engine protection.
3. Strongest oil film – increased power and wear protection.
4. Highest flash point – reduced oil consumption.
5. Best oxidation resistance – preservation of the oil’s core properties throughout the entire service interval.
Well, once again we remind about the drawbacks, one of which has turned into a myth. Namely – loss of basic properties and fluidity when ester compounds interact with water. In this case the ester oil turns into jelly. Shocking pictures and warnings appear on forums, scaring car owners about “treacherous” ester oils that fail when a drop of water gets in. But we disappoint the “detective‑genre” enthusiasts. To bring an ester oil to such a state, you would need a volume of water equal to the volume of oil. It is absurd for such an amount of moisture to accidentally enter the system. And a small amount of condensation that may form in the system due to temperature differences is completely harmless. It evaporates quickly as the oil reaches operating temperature and is vented through the crankcase ventilation system.
And, as mentioned, the main drawback of an ester base is its cost. A 100 % ester base is a theory, not a practice. Yet even a small amount of this component endows the oil with all the properties we discussed above, to varying degrees. The ester content in engine oils is usually limited to a few percent (rarely more than 10 %), and they are used only in the most advanced products that typically sit at the top of the product range of leading companies.
Take note! The word “ester” in chemistry is as broad a concept as “alcohol”. Alcohol can be food‑grade ethyl or poisonous methyl (also called wood alcohol). A typical ester in oil is simply the acid part of a carboxylic acid combined with some hydrocarbon group. The children’s favorite toy – nitroglycerin – is also an ester, but based on nitrogen rather than carbon. In general, you can type the name of most well‑known oil brands into Google together with the word “ester” and see that almost everyone uses esters. The only thing that can favorably differentiate an expensive racing oil from a cheaper conventional one is which specific esters are blended, because their lubricating and film‑forming properties can differ significantly.
Let’s give examples. A prominent manufacturer of ester oils is the Belgian company XENUM. Five of the company’s products – Xenum WRX 7.5W40, VX 5W30 and the X1 Ester Hybrid oil series in viscosities 5W‑30, 5W‑40, 5W‑50 – contain esters in their formulation.
If you look at the “dry numbers”, they are unlikely to spark the imagination. Pour points and flash points, viscosity values at the main reference temperatures (40 °C and 100 °C), HTHS figures (high‑temperature viscosity at 150 °C) – all of these can fall within the same ranges as those of the PAT‑brother brands. The key metric is property stability! And that can only be determined through analysis of used oil.
But no numbers will show you the “adhesion effect”. Perhaps only an increased engine life, which you will notice much later. It should also be remembered that the more esters an oil contains, the more stable its viscosity remains! The presence of esters enables the aforementioned Xenum oils to contribute to fuel savings (6‑19 %), increase engine power (3‑6 %), and reduce noise and component wear.
So, should you buy expensive ester oils? That’s a personal decision for each individual. However, based on everything said above, one can state unequivocally – ester oils are for those who did not buy a car with a 2‑3‑year resale horizon. They are more suited to owners who keep a vehicle for the long term and view oil purchase and replacement not just as another service interval, but as a reliable investment in engine longevity.
