Motor Oil Viscosity, Why it's Important

What is viscosity? When a fluid is subjected to external forces, it resists flow due to internal molecular friction. Viscosity is a measure of that internal friction. Viscosity can be referred to as the measurement of a fluid’s resistance to flow.

Viscosity can be viewed in two different ways. The first is a fluid’s tendency to flow as it is visually seen. One can think of this as the time it takes to watch a fluid pour out of a container. The term for this kinematic viscosity and it is expressed in units suggesting flow volume over a period of time. The most commonly used unit of kinematic viscosity is centistokes (cSt). Another is a Sabolt Universal Second (SUS or SSU). When comparing the viscosity of two different fluids, keep in mind that a centistokes and Sabolt Universal Second are different units, and therefore they cannot be directly compared. The temperature of the fluid being tested will also affect the outcome. Kinematic viscosity of a lubricant is determined at 100C (212F) and/or 40C (104F) depending on the grading system being used. For general comparison purposes, the temperature at which the viscosity was determined for the two products must be the same.

The second is a fluid’s tendency to flow as is indicated by measured resistance. You can think of this as the energy required to move an object through a fluid. It takes little energy to stir water with a spoon. However, significantly more energy is required to stir honey with that same spoon. The term for this is dynamic viscosity or absolute viscosity and is expressed in units known as centipoises (cP).

Whether it is reported in centistokes, SUS or centipoises units, the higher the number assigned, the more viscous or thicker the fluid is. And a fluid’s viscosity or thickness is directly proportional to its internal friction and resistance to flow.

There are other ways to refer to a fluid’s viscosity.  Some of the more common terms are thin, light or low. These terms suggest how a relatively free-flowing fluid such as water flows. Terms such as thick, heavy or high suggest that the fluid demonstrate a strong resistance to flow. Honey is a good example of a fluid with a high viscosity.

A fluid’s viscosity is important because it is directly related to its load-carrying capabilities. The greater a fluid’s viscosity, the greater the loads it can withstand. The viscosity of a fluid must be adequate to separate moving parts under normal operating conditions (temperature and speed).

Knowing that a fluid’s viscosity is directly related to its ability to carry a load, one would think that the more viscous a fluid, the better it is. The fact is, the use of a high-viscosity fluid can be just as detrimental as using too light an oil.

Too low (thin or light) = Metal-to-metal contact (friction and wear), poor sealing and increased oil consumption

Too high (thick or heavy) = Increased fluid friction, Reduced energy efficiency, higher operating temperature and equipment starting difficulties particularly at cold temperatures.

The key is to select a fluid that is not too light and not too heavy.

Fluids (lubricant stocks) thicken as they are cooled. As their temperature continues to decrease, they will eventually reach a point at which they become no longer fluid. As they thicken, their load-carrying ability increases, but their ability to be circulated becomes significantly impaired. As fluids are heated, they thin, which reduces their ability to prevent metal-to-metal contact. Therefore, it is important that equal temperatures be used when discussing or comparing the viscosity of fluids.

SAE Single-grade (sometimes referred to as straight grade or mono-grade) oils indicate their viscosity performance at either high or low temperature. They are designed for use when operating temperatures are relatively constant. Examples of high-temperature, single-grade motor oils would SAE 30, 40 or 50 and SAE 80, 90 or 140 for gear lubricants. Examples of single-grade oils with low-temperature performance are SAE 5W, 10W or 15W for motor oils and 75W, 80W and 85W for gear lubricants. The presence of a W in oil’s grade (such as 10W or 80W) indicates low-temperature performance. Anytime you see the W, think of it as standing for winter or cold.

Multi-grade oils are concerned with performance at both high and low temperatures. They are designed for use when operating temperatures vary significantly. For example, if a trucker leaves Florida with a load of oranges bound for Minnesota, the truck’s engine may have SAE 30 single-grade oil in it, which is appropriate for the ambient conditions in Florida. Upon reaching the subzero temperatures of Minnesota, the driver finds that the SAE 30 is too viscous to allow for starting of the vehicle. By draining the SAE 30 and installing an SAE 15W, he finds his staring problems are resolved and he continues his work. However, returning to Florida causes the SAE 15W to thin to the point that engine damage occurs.

This could be avoided by changing back to higher viscosity oil prior to a return to the warmer climate. The need for such frequent oil changes is not only inconvenient; it raises the cost of operation. Multi-grade oils allow for operation in changing climates without the need for changing the engine oil. Typically multi-grade engine oils are 5W-30, 10W-30 15W40 and 20W-50. Typical multigrade gear lubricants are 75W-90, 80W-90 and 85W-140.

How are multigrade (multi-viscosiy) oils made? Some exist naturally. For example, many synthetic base stocks have sufficient viscosity stability (index) to qualify as multi-grade oils. For mineral base oils, it is another story. They are created by first selecting a base stock that possesses the desired low-temperature properties. To that stock, chemical compounds known as viscosity index improvers are added. These viscosity index improvers minimize the thinning of the oil as temperature increase. This allows the lighter base sock to now also meet the higher temperature requirements.

When selecting a lubricant, there are three basic rules of thumb to consider:

• The lower the starting and/or operating temperature, the lighter or thinner the selected oil should be. The higher the staring or operating temperature, the heavier or thicker the selected oil should be.

• The higher the load a component is subjected to, the heavier or thicker the oil should be. The lighter the load a component is subjected to, the lighter or thinner the oil should be.

• The faster the operating speed for a piece of equipment, the lighter or thinner the oil should be. The slower the operating speed for a piece of equipment, the heavier or thicker the oil should be.

  There is no advantage in using heavier oil than is needed. In fact, it can be a disadvantage.

  Remember, too low (thin or light) = Metal-to-metal contact (friction and wear), poor sealing and increased oil consumption. Too high (thick or heavy) = Increased fluid friction, reduced energy efficiency, higher operating temperatures and equipment starting difficulties particularly at cold temperatures.