The earliest attempts to classify motor oils were made when automobiles first appeared. Even at this early stage, viscosity was recognized as one of the most important characteristics of oil. For this reason, the Society of Automotive Engineers (SAE), in co-operation with engine manufacturers, developed the original SAE J300 viscosity grading system for engine oils way back in 1911.

Oils were assigned numbers based on viscosities at certain temperatures. Over the years these standards were updated several times to keep in pace with engine developments and technology advancements.

It has been recognized that oil viscosity at colder temperatures, as well as at high operating temperatures, is very important in the performance of an engine. The SAE has therefore devised two separate viscosity measurement systems, one at a high temperature (100°C) and one at very low temperatures. A rotating viscometer, called a cold cranking simulator, is used to measure viscosities at temperatures as low as -35°C. Because the viscosities are measured in two different temperature ranges, the results are reported in two different units.

The first unit is the centipoise (cP). It is used to report the absolute viscosity of motor oil at low temperatures. This number indicates the ease with which the oil can flow when cold. The other unit is the Centistoke (cSt) which is used to report the kinematic viscosity of motor oil at higher temperatures.

Oils that are suitable for use in colder temperatures are identified by the letter “W” when indicating the SAE viscosity grade. These oil grades must meet maximum viscosity limits at specified sub-zero temperatures and must also meet maximum requirements for the borderline pumping temperatures at very low temperatures. Oils that are suitable for use at higher temperatures have viscosities within specified ranges at 100°C. The standards below have been used to classify engine oil viscosities for a number of years:

SAE BOO Engine Oil Viscosity Grades

If we draw graphs of typical SAE 5W and SAE 40 monograde oils with viscosity plotted as a logarithmic function on the vertical axis against temperature as a linear function on the horizontal axis, we will end up with the two solid red lines in the diagram below:

The SAE 5W oil will flow sufficiently at low temperatures to protect engines during startup on cold mornings but will be too thin to provide adequate protection at operating temperatures. The SAE 40 oil on the other hand will perform satisfactorily at operating temperatures but will be too viscous to flow sufficiently during startup on cold mornings. The solution? An oil that is ‘thin’ on cold mornings but with a viscosity similar to that of a SAE 40 at operating temperature. But how do we achieve that? With a viscosity modifier (viscosity index improver).

A viscosity modifier (VM) is an oil additive that is sensitive to temperature. At low temperatures, the VM contracts and does not impact the oil viscosity. At elevated temperatures, it expands and an increase in viscosity occurs. If we use a thin oil (let’s say the SAE 5W above) as base and add sufficient VM to meet SAE 40 viscosity limits at 100°C, we end up with a SAE 5W-40 multigrade oil – the red dotted line. Similarly, there are SAE 15W-40, SAE 20W-50, etc. multigrade engine oils available in the market. Multigrade oils provide better engine protection at low and high temperatures than monograde oils because they maintain optimum viscosity over the full engine operating temperature range.

Of particular interest is the inclusion of three new high temperature viscosity grades in the latest revision of the SAE J300 Engine Oil Viscosity Classification Standard. They are SAE 16, SAE 12 and SAE 8 (not shown in the table above). These new grades reflect the continued industry push for lower viscosity engine oils to achieve improved fuel economy. They establish specifications to standardize new lower viscosity lubricants such as SAE 5W-12, or even SAE OW-8, in the marketplace.

Viscosity  is probably the single most important property of oil in terms of lubrication but what is viscosity really?

Informally viscosity is the “thickness” of a liquid. For example, if you pour water into a container with a hole at the bottom, the container drains quickly.

However, if you fill the same container with honey, you will find the container drains very slowly. That is because the viscosity of honey is high compared to that of water. We can therefore say that viscosity is an indication of a fluid’s resistance to flow.

More formally, viscosity is a measure of the internal friction of a moving fluid. Most liquids are both cohesive and adhesive. Cohesiveness is the intermolecular attraction by which the molecules of the fluid are held together and result in internal friction. A fluid with low viscosity flows easily because its chemical structure results in very little friction when the oil molecules are in motion.

Imagine you have two horizontal plates or metal surfaces with oil in-between. The oil will cling to the two surfaces because it is adhesive. If the top plate moves horizontally relative to the stationary bottom plate the speed of oil molecules in between will vary from zero at the bottom to the same speed as the top plate.  As the oil layers slip over each other they create friction as a result of the cohesiveness of the oil molecules.

The most commonly used unit for measuring viscosity is the Centistoke (cSt). Viscosity is frequently measured using a device called a capillary viscometer – a U-shaped, graduated glass tube with a capillary of known diameter in the one arm. This method measures the time taken for a defined quantity of fluid to flow through the capillary.

When two fluids of equal volume are placed in the same viscometer and allowed to flow under the influence of gravity, a viscous fluid takes longer than a less viscous fluid to flow through the capillary and a higher viscosity is recorded.

Since this method uses gravity as the driving force; the result is kinematic viscosity.  The metric unit of kinematic viscosity is mm²/s (2cSt = mm²/s). One disadvantage of the capillary viscometer is that the capillary is too small for highly viscous liquids.

From everyday experience, it is common knowledge that viscosity varies with temperature. Honey flows more readily when heated. Likewise, oils thicken noticeably on cold days with a resultant increase in viscosity. Since viscosity is so dependent on temperature, it should NEVER be quoted without reference to the temperature at which it was measured. Kinematic viscosity is generally measured at 40°C and 100°C.

The low-temperature characteristics of certain lubricants are important to their proper operation. Measurement of the sub-zero viscosity of automatic transmission fluids. Engine oils, etc. is often used to specify their suitability for service.

To measure the viscosity of oils at low temperature, dynamic (or absolute) viscosity is often employed using a Brookfield viscometer. Brookfield viscometers rotate a spindle (at a defined speed) in the viscous cold oil and measure the torque required to rotate the spindle in the oil and report viscosity values using the centipoise (cP) or milliPascal-second (mPa-s), as the unit of viscosity (1 cP – 1 mPa-s).

Another key property of lubricating oil is Viscosity Index (VI). It is an arbitrary measuring scale (without units) that indicates the change in oil viscosity with change in temperature. For example, honey is thick at room temperature but when you heat it to say 60°C, it flows readily because the viscosity is reduced significantly and thus has a low VI.

However, there is hardly any visible change in the viscosity of water from room temperature to 60°C and we can therefore say water has a high VI compared to honey. Failure to use on oil with the proper VI when temperature extremes are expected may result in poor lubrication and equipment failure.

And finally, oil viscosity selection. When choosing an oil for a specific application the first consideration should always be an oil with a viscosity that is sufficient to keep the metal surfaces apart. Unfortunately, viscosity cannot be considered in isolation. Selection of the correct viscosity will depend on the temperature, load and speed encountered in a specific application.

Temperature:

For machines operating under constant load, constant speed and constant ambient temperature such as an industrial gearbox in a factory, the ideal viscosity very often results in the lowest stabilized oil temperature. Oils of lower or higher viscosities (than the optimum viscosity) will typically increase the oil’s stabilized temperature due to either drag/churning losses (too much viscosity) or mechanical friction (too little viscosity).

If conditions are not constant (variable loads, changing speeds, extreme temperatures, etc.), then there is a need for not only the optimum viscosity but also a high viscosity index to stabilize the optimum viscosity. The wider the temperature range, the greater the need for higher VI oils.

Load:

Operating conditions determine the load on machinery. The load on an engine in a vehicle under acceleration or going uphill is higher than that of a vehicle cruising down the highway. Load refers to the pressure on the moving surfaces.

Effective lubrication means being able to separate the load carrying surfaces and, if the load changes, then the optimum viscosity of the oil required to separate the surfaces can change. If the load is too high, the oil film may be squeezed too thin to protect the metal surfaces from making contact. This will result in solid friction, meaning an increase in heat, wear and ultimately machine failure.

Speed:

The faster a machine operates the stronger the oil film will become as more oil is dragged into the area between the metal surfaces. Therefore, for high speed applications, a low viscosity oil is required. Conversely, for low speed applications, a high viscosity oil needed to maintain a solid oil film and separate moving surfaces.

In summary, oil viscosity must be sufficient to keep metal surfaces apart yet it must not be so viscous that it will increase drag and waste energy.

In most instances lubricating oil is a blend of base oil and additives with the base oil content being anything between 70 percent and more than 99 percent depending on the final application of the lubricant. Base oils may be mineral, synthetic or semisynthetic – a mixture of mineral and synthetic stocks. Most lubricating oils used globally (more than 90 percent) are blended using mineral base oils. Feed stocks from a number of streams at crude oil refineries are processed at base oil refineries to produce various viscosity grades of mineral base oils. A typical mineral base oil refinery will have the following units to produce suitable quality base oils:

• Solvent Extraction to remove undesirable aromatic (unsaturated) compounds which are unstable and cause the formation of tar, varnish and carbon in engines.
• Propane De-asphalting removes asphaltic material from the base stocks to minimize the formation of deposits in machinery, and
• Dewaxing to improve low-temperature fluidity of the base oil.

These three (extraction) conversion processes generally produce Group 1 base oils with aromatic content between 15 and 20 percent. The colour of Group 1 base oils would normally vary from a light yellow to straw. The quality of such base oils can be further improved by a number of Hydrofinishing Processes. Hydrofinishing changes the remaining unsaturated/aromatic compounds in the oil by a chemical reaction involving hydrogen and produces base oil with improved chemical stability, lower sulphur content and much lighter colour. The final quality of the base oil is determined by the severity of the application, temperature and pressure in the hydrofinishing process and will normally be classified Group 2 or Group 3 base oils. The quality and characteristics of modern Group 3 base oils approach that of synthetics.

Synthetic base oils are manufactured from chemical building blocks and excel mineral oils in viscosity index, shear stability, low and high temperature performance, oxidation stability and volatility. A major disadvantage of synthetics is that they cost approximately 3 to 5 times more than mineral oils. They therefore tend to be used in specialty applications only where the performance of mineral oil is considered unsatisfactory. Typical examples are very high temperature applications and extended oil drain intervals.

The most commonly used synthetic oil is polyalphaolefin (PAO). PAO’s are classified Group 4 base oils and are used in a wide variety of automotive and industrial applications such as engines, transmissions and hydraulic systems. The use of Group 5 base oils (typical synthetic esters) are limited to very special applications such as refrigeration compressor oils and aviation turbine lubricants. The table below shows the general differences between the various Groups.

Mineral and synthetic base oils are produced in a number of viscosity grades. For instance, low viscosity (thin) base oils would be used to produce automatic transmission fluids whilst thick, heavy ones are required to blend ISO 680 viscosity grade gear oils.

A final word of advice: avoid mixing different oil Groups. In an emergency situation, mineral oils may be mixed with PAO’s, but Group 5 synthetics should preferably not be used with any other oil Group.

All Q8Oils lubricants produced by Blue Chip Lubricants are blended exclusively from imported Q8 base oils that are manufactured from Kuwait Export Crude. Most petroleum products are derived from crude oil which, as a natural material, has a tendency to vary in type and quality depending on its source. Kuwait Export Crude is however unique, its consistency and superior quality make it the perfect feed-stock for refining high quality base oils. Q8 base oils are hydro-finished using a unique process that removes any remaining impurities rendering them clear, pure and very oxidation stable.

With complete control of our raw materials we can guarantee a consistency of product quality matched by few other companies and our customers can have complete confidence in the performance of our products.