What is the difference between synthetic, semi-synthetic and mineral oil?

The difference is in the base oil, the liquid that makes up almost all of a lubricant: mineral comes from refining crude oil, with molecules of many shapes, and synthetic is made of molecules built or reworked to be more alike. That is why the most common synthetic evaporates less, flows better in the cold and, with the right additive, lasts longer in the heat. Semi-synthetic mixes the two and, in Brazil, needs at least 10% synthetic to carry the name, under the rule of the National Petroleum Agency (ANP).

What is a lubricating oil made of?

Of two parts: the base oil, which is almost the whole volume, and the additives, which are the smaller part.

Think of a cake. The base oil is the batter; the additives are the yeast and the seasoning, which go in small amounts and change the result.

An additive is a chemical, used in a small dose, that gives the oil a quality it would not have alone, or strengthens one it already has, such as protecting parts against wear and rust. How each base oil is produced is in the guide to what base oils are.

Mineral, synthetic and semi-synthetic are names that describe only the base oil. The additives are present in all three, and the same name on the label does not guarantee the same additive package.

One word will come up often: viscosity, how much the oil resists flowing, its "thickness". Honey has high viscosity; water has low viscosity.

Now the base oils, starting with the oldest and most common, mineral.

What is mineral oil?

It is a lubricant made with base oil taken from crude oil.

Picture a wall of river stones. Each stone has its own size and strength, and when time takes its toll, the weaker ones give way first. A mineral base is like that on the inside.

Crude oil is a mixture of thousands of molecules, almost all made of carbon and hydrogen: the hydrocarbons. The refinery separates the heavy part, which becomes base oil, and cleans it: it removes part of the aromatics (with solvent, in the simplest refining) and part of the wax (dewaxing).

Even after cleaning, this part still has molecules of three shapes, the families explained in the text on paraffinic, naphthenic and aromatic oil:

• Paraffinic: carbon chains, straight or branched. The long straight ones form wax in the cold.

• Naphthenic: carbon rings with single bonds only, which are stable.

• Aromatic: rings with alternating double bonds, like the one in benzene; those with several rings, and those with a chain attached to the ring, are the ones that react most easily.

Alongside them remain sulfur and nitrogen compounds, in an amount that depends on the refining.

The more the refinery cleans, the more the oil ends up with saturated molecules, which have all the hydrogen they can hold. They break down more slowly in the heat and leave less dirt. And the oil ends up with fewer aromatics and less sulfur.

Base oils are divided into five groups by the classification of the American Petroleum Institute (API), the United States oil industry association. It is this cleaning that separates Group I from Group II.

Group II goes through treatment with hydrogen under pressure (hydrotreating or hydrocracking), which removes almost all the aromatics and sulfur; the difference is covered in the text on Group 2 base oil.

For those who need the numbers: under API 1509 (Annex E), Group I has less than 90% saturates or more than 0.03% sulfur; Group II has 90% or more saturates and 0.03% or less sulfur. Both fall in the viscosity index range of 80 to less than 120, a number explained further on.

Resolution ANP 804/2019, art. 4, item X (the Brazilian regulator's rule that defines lubricant types), uses the same numbers and counts as mineral the Groups I and II and the naphthenic bases (with ring-shaped molecules and no aromatics), among others. The groups come back just below, in the synthetic section.

And the synthetic, what is it like on the inside?

What is synthetic lubricating oil?

It is a lubricant made only with synthetic base oils, whose molecules are assembled in a reactor or reworked until they are more alike than those of mineral oil.

If a mineral base is the wall of river stones, a synthetic base is the wall of factory bricks, with pieces that are much more alike.

One example is PAO (polyalphaolefin), Group IV. The petrochemical plant joins small, identical molecules in a reactor until they reach the size it wants. The result is a base made almost only of carbon and hydrogen, like mineral oil, but with no aromatics, no sulfur and no wax.

The ANP calls synthetic the API Group IV, which is PAO, and other bases made in a reactor, such as esters. It also counts Group III as synthetic, which comes from crude oil; that catch appears just below.

Figure 1. Inside the two bases: the mineral one gathers short and long chains, straight and branched, rings and sulfur compounds; the PAO synthetic one has branched molecules that are much more alike.

There are other synthetic bases, such as esters (made from an acid and an alcohol) and polyglycols, or PAG, used in special services. They come back later, only as far as they matter for the choice.

The label catch. Group III is almost always born from crude oil, like mineral oil. But it goes through a treatment with hydrogen at high pressure, harsher than the one for Group II: hydrocracking with isomerization, which breaks the molecules and rearranges straight chains into branched ones.

Like Group II, it comes out almost entirely as saturated molecules with little sulfur. What changes is the shape: branched chains with few rings, which make the viscosity change less with temperature. That is what takes it out of Group II. The ANP counts it among the synthetics, and the market sells it as synthetic.

The result: two oils with "synthetic" on the label can have a Group III or a PAO base. Ask the supplier which one it is.

For those who need the numbers: synthetic base oils are in Resolution ANP 804/2019, art. 4, item XI, and the composition of the groups is in item IX. Under API 1509 (Annex E), Group III has 90% or more saturates, 0.03% or less sulfur and a viscosity index of 120 or more; Group V is defined by exclusion, with no list of names.

In the ANP list (item IX), naphthenic and white mineral oil (a highly refined oil, almost only saturates, with no color or smell) appear in Group V, the one for all other bases. Naphthenic usually goes there because its viscosity index is below 80.

Group and nature are two different lists: the group says what the base is made of, and the nature says whether the lubricant is mineral, semi-synthetic or synthetic.

With both bases presented, the mix of the two is easy to understand.

What is semi-synthetic oil?

It is a lubricant that joins mineral base oil and synthetic base oil, with at least 10% synthetic, counted by weight.

Think of a latte, where the coffee is the synthetic and the milk is the mineral. How much coffee must the cup have to be called a latte? For lubricants, the ANP drew that line.

Resolution ANP 804/2019 separates lubricants into three natures:

• Mineral: almost entirely mineral base oil, with less than 10% synthetic.

• Semi-synthetic: mineral and synthetic together, with 10% or more synthetic.

• Synthetic: no base oil other than synthetic ones.

The calculation is made by weight (mass), not by volume.

Figure 2. Where semi-synthetic begins: with 10% synthetic base oil by mass the oil already carries that name, and with 60% it keeps it; only at 100% is it synthetic, and the ANP rule sets only the floor.

A semi-synthetic with 10% synthetic and another with 60% carry the same name, and neither is required to say how much it contains.

The Group III catch comes back here. Since the ANP counts it as synthetic, a semi-synthetic can be made only with base oils that came from crude oil.

To compare two semi-synthetics, ask the supplier which base oils each one is made of and what use it is recommended for. The comparison is made by use, not by name.

For those who need the numbers: the three natures are in Resolution ANP 804/2019, art. 4, items VI to VIII. Resolution ANP 941/2023 deals with the producer's authorization and does not change these definitions.

One basic question remains: why does a synthetic base make the oil last longer?

Why does synthetic oil withstand more heat and cold?

Because, in a mineral base, some types of molecule are the weak point, and heat and cold find that point first.

It is like a chain, which breaks at its weakest link. These are the weak links of a mineral base:

1. The shorter, lighter molecules evaporate first when the oil heats up, which is called volatility. This leads to topping up oil more often and to an oil that thickens over time, because the heavy part stays behind.

2. Multi-ring aromatics and nitrogen compounds are among the first to react with oxygen in the air, which is oxidation. This leads to acids, sludge (a pasty deposit that clogs filters) and varnish (a hard, dark film on parts).

3. In the cold, long straight paraffinic chains clump into wax crystals, which makes the oil thicken and stop flowing. The lowest temperature at which it still flows is the pour point.

4. Aromatic and naphthenic rings change viscosity more with temperature than paraffinic chains do, which leads to a lower viscosity index. This index tells how much the viscosity changes when the temperature rises or falls: the higher it is, the less the oil thins in the heat and thickens in the cold.

At the same viscosity, PAO has molecules of more similar size, with almost no molecule much shorter than the others, and it has no aromatics, sulfur or wax. That is why it evaporates less, flows in the cold and has a high viscosity index.

As for oxidation, the advantage shows in the oil already with its additives, the finished oil ready for use. With the right antioxidant, the additive that slows oxidation, the PAO-based oil lasts longer in the heat than the mineral-based one.

For a fair comparison, both oils must have the same grade, the same viscosity range at 40 °C; that is how Figure 5 compares them.

Figure 3. As the oil heats up, in the mineral base the short molecules evaporate first and the aromatics oxidize first, while the naphthenic ring resists better than the aromatic; in the PAO base, with more alike molecules, the oil evaporates less and, with the right antioxidant, oxidizes more slowly.

Figure 4. In the cold, the long straight chains of the mineral base clump into wax and the oil stops flowing, which is the pour point; in PAO, with no wax, the oil keeps flowing.

Figure 5. Two oils of the same grade sit near the same viscosity value at 40 °C; away from that temperature, mineral oil thickens more in the cold and thins more in the heat than synthetic.

Group III sits between the two: much better than Groups I and II in evaporation and viscosity index, below PAO in the most severe cold.

How this shows up in your plant: oil top-ups above normal on equipment that runs hot, oil darkening before its time, varnish on valves and a filter that saturates early. In oil analysis, acidity rises fast; in the cold, the pump or compressor is slow to start.

For those who need the numbers: by the rule of thumb derived from the Arrhenius law, which is only approximate, in mineral oil above about 60 °C each extra 10 °C cuts the oil's oxidation life by roughly half. The reason is that the reaction rate doubles.

Volatility is measured by the Noack method (ASTM D5800, NBR 14157); the pour point, by NBR 11349 (ASTM D97); and the viscosity index, by NBR 14358 (ASTM D2270), calculated from the viscosities at 40 °C and at 100 °C.

So is synthetic always better? The answer depends on the machine.

When does synthetic oil pay off in industry?

It pays off when the cost per machine hour goes down. That depends on temperature, the interval between changes and the cost of stopping the machine, more than on the price per liter.

Think of a road tire and a racing tire. The racing tire is better, but it only pays off on the track.

Synthetic usually pays off when at least one of these conditions exists:

• the oil runs hot and mineral oxidizes before its time, with acidity rising early in the analysis;

• the change requires an expensive stop, a crane or difficult access, as with the gearbox at the top of a wind turbine;

• the machine starts in severe cold, as in a cold room, and mineral oil is slow to reach the parts;

• maintenance can take advantage of the longer interval between changes (if the change stays on the usual calendar, the synthetic's gain is lost).

An example: in a screw compressor that runs all day, PAO synthetic oil usually allows longer intervals between changes than a mineral. That means fewer changes, fewer stops and less oil to dispose of, as long as the oil analysis confirms it.

Synthetic does not pay off on a machine that runs at normal temperature and load, with a quick, cheap change. There, a well-refined mineral does the job and costs less. And no base oil fixes the wrong viscosity grade, water in the oil or an expired filter.

The Compstar line, from Kelpen Oil, has mineral, semi-synthetic and synthetic options for compressors. The full choice is in the guide on which oil to use in an air compressor; the one for gearboxes, in which oil to use in an industrial gearbox.

Synthetic also has limits:

• PAO dissolves little additive and, alone, shrinks certain sealing rubbers. That is why the formula includes a little ester (another synthetic base) or an additive that swells the seal in the right measure.

• Some esters react with water and break down into acid and alcohol, which is hydrolysis, when the system has moisture.

With that, the choice can be made machine by machine.

Which is the best oil: synthetic, semi-synthetic or mineral?

The best is the one that covers the machine's temperature, change interval and cost of stopping. Paying for resistance the machine does not use is cost with no return.

And if the machine already has mineral oil and you want to switch to synthetic, or top up with what you have on hand?

Can you mix synthetic oil with mineral oil?

It depends on which synthetic.

PAO and mineral have the same chemistry, both made only of carbon and hydrogen (hydrocarbons), and in general they mix without separating. The mix, however, dilutes the resistance you paid for in the synthetic, and the additives of the two oils may not match.

Common PAG is different: it generally does not mix with mineral oil or with PAO, and the change calls for its own procedure.

Esters and pure PAO can affect certain sealing rubbers: ester tends to swell the rubber, and pure PAO to shrink it, depending on the rubber type. That is why the formulation balances the bases. If in doubt, talk to the Technical Team.

The practical rule is simple. When changing from one to the other, drain, clean and fill with the new charge; the Turan S line asks for this when replacing a mineral oil. Day to day, top up the level always with the same product that is in the machine.

How can you tell if an oil is synthetic or mineral?

By the label, by the supplier and, when the doubt is serious, by the laboratory:

1. Label: under the current rule, lubricants that need product registration with the ANP, such as engine oils, state the nature on the label.

2. Supplier: ordinary industrial oil is outside that registration. Ask which base oil it is made of: Group I or II mineral, Group III (which the ANP counts as synthetic) or another synthetic base.

3. Laboratory: a set of tests gives strong indications of the base family, and confirmation comes from the supplier.

We at Kelpen Oil have been a reference in base oil distribution since 1999. We distribute Group II (HBP-I line) and Group III (HBP-S line) base oils to formulators, in the category of base oils for lubricant producers.

We also formulate finished lubricants in all three natures.

In our own laboratory in Arujá (SP), the difference between the bases shows up in the tests in the table below; the role of each test is in the text on the lubricant laboratory. None of these tests, alone, separates a well-treated Group III from a PAO.

In oil in service, tracking acidity and viscosity over time shows whether the synthetic is lasting as long as it should.

For those who need the numbers:

Frequently asked questions

Is 20W50 oil synthetic or mineral? The SAE grade, such as 20W50 or 15W40, gives the engine oil's viscosity in the cold and in the heat, and the same grade can be sold as mineral, semi-synthetic or synthetic. The nature is on the label, which the current ANP rule requires on engine oil.

Which is the best engine oil, synthetic or mineral? The one the manual asks for: the SAE grade and the performance level, the engine oil quality classification, such as the one from ACEA. A synthetic that meets the grade and level requested can replace mineral; the reverse only holds if the mineral meets the same grade and the same level.

Is synthetic oil thinner than mineral? At the same grade, both sit in the same viscosity range at 40 °C, because the ISO VG grade, the industrial oil thickness measured at 40 °C, is the same for both. An ISO VG 68 falls between 61.2 and 74.8 cSt (centistokes, the unit of viscosity), mineral or synthetic (ISO 3448). The difference shows up away from 40 °C, as in Figure 5: in the cold the synthetic thickens less, and in the heat it thins less.

Does synthetic oil come from crude oil? In large part, yes. PAO is made from 1-decene, which the petrochemical industry produces from ethylene, taken from crude oil or natural gas; Group III comes from crude oil refining or, in some cases, from natural gas. What changes is the process, which makes the molecules more alike. Some esters, on the other hand, use plant-based raw material.

Glossary

• Viscosity: how much the oil resists flowing; honey has high viscosity, water low.

• API groups: the division of base oils into five groups (I to V) by composition; the ANP counts Group III as synthetic.

• PAO (polyalphaolefin): Group IV synthetic base, made in a reactor by joining small, identical molecules.

• Viscosity index: how much the viscosity changes with temperature; the higher it is, the less the oil thins in the heat and thickens in the cold.

• Oxidation: the oil's reaction with oxygen in the air, which forms acids, sludge and varnish and speeds up with heat.

Talk to the Technical Team

The Kelpen Oil Technical Team looks at the real oil temperature, the change interval you use, the cost of stopping, the machine's seals and the analysis history, and recommends the right nature, mineral, semi-synthetic or synthetic. If the oil is already in service, the Arujá (SP) laboratory performs the in-service oil analysis, as a paid service.

Before paying for synthetic, note the oil temperature, what a stop costs and how often you change the oil today. Save this article or send it to whoever buys lubricants at your company.