Oil Tech: Viscosity

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Apr2012 25

One property of engine oil that most people are familiar with is the grade. Oil grade is sometimes referred to as weight, and oil grades are measured using a standardized test developed by the Society of Automotive Engineers (SAE) and are outlined in the document SAE J300.

SAE J300 defines separate testing methods for standard weights (20, 30, 40, 50, and 60) and winter weights (0W, 5W, 10W, 15W, 20W, 25W) based on the oil’s performance without viscosity modifiers. Standard oil weights are determined based on the kinematic viscosity low shear rate at 100 °C , as well as the minimum dynamic viscosity high shear rate at 150 °C. The winter grades are based on a test designed to simulating cold cranking and the grade is determined based on the lowest temperature it meets the dynamic viscosity criteria, with lower grade numbers relating to colder temperatures. This is why you would use a 5w instead of a 10W in colder climates. Multi-grade oils weights (ie, 5W40, 10W30, etc) are used to designate oils that meet both the standard and winter weight criteria as outlined in SAE J300. It should also be noted that the criteria for grading gear oils (75W90 and the like) use a completely different scale.

So, what is viscosity? Dynamic viscosity, or absolute/simple viscosity, is the measure of a fluid’s ability to resist deformation in the presence of shear stress, or the resistance to movement of the layers of a fluid when subjected to a force. Kinematic viscosity is defined as a measure of a fluid’s resistance to flow or deformation, or, essentially, the thickness of a fluid. The higher the kinematic viscosity, the more resistant it is to flow, thus, the slower it will flow. For example, molasses or honey would have a higher kinematic viscosity than, say, water. Kinematic viscosity is also defined as the dynamic viscosity divided by the density.

Another important variable regarding engine oil, also developed by SAE, is viscosity index. Viscosity index is an arbitrary measure of how the viscosity changes with temperature. Without getting into too many details, it is a number from zero (worst) to one hundred (best), although advances in lubricant technology have led to the formulation of oils have a viscosity index over one hundred, thanks to viscosity modifiers.

As you probably assumed, viscosity modifiers improve the viscosity index of engine oils. Most multi-grade oils use viscosity modifiers to allow good cold temperature (cranking) flow while not allowing them to thin out at high temperatures. It could be said that a 5W40 oil essentially acts like a 5W weight oil when cold that will not thin out more than a 40 weight oil at temperature. Viscosity modifiers are typically certain types of polymers or olefins.

Another viscosity-changing oil additive that is sometimes used is called a pour point depressant. This improves the cold flow characteristics of the oil. Specifically, it decreases the lowest temperature at which the oil will flow, which improves the cold-starting/cranking flow of the oil. This is used because petroleum-based oils contain paraffin (waxes) that solidify/crystallize at low temperatures. Pour point depressants inhibit wax precipitates from forming by inhibiting crystal formation. Some common pour point depressants are alkylated napthalene (similar to the stuff in mothballs), esters, phenols, or certain polymers.

Considering how important the viscosity of your engine oil is to maintaining proper oil pressure and resisting wear of engine components, it is not surprising that so much technology goes into precisely dialing in the oil’s viscosity over the entire range of temperatures you would expect to see, from when you first crank all the way to operating temperature. Viscosity is just one characteristic related to your oil accomplishing the complex range of duties expected of it on a daily basis.

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