What are the advantages of Multi Viscosity Car Engine Oils?

A multi grade car engine oil is one whose properties have been artificially modified to reduce changes in its viscosity with temperature changes. A multi grade car engine oil will be more fluid at low temperatures and thicker at high temperatures than a mono grade car engine oil.

That means that when the vehicle is not being used (for several hours), the engine oil sinks in the crankcase so that, on starting, it takes a certain amount of time (a few microseconds) to again reach all parts of the engine that need to be lubricated. Since a multi grade engine oil is more fluid at low temperatures, it reaches the various engine components more quickly than a mono grade oil, thereby reducing wear on starting.

Multi grade engine oils usually produce fuel savings of 1.5 to 3% in comparison to mono grade oils.

Multi grade oils provide better engine protection at low and high temperatures than monograde oils, in that they maintain optimum viscosity over car engine operating temperature range.

How do Multi Viscosity Oils Work?

A VI (Viscosity Index) improver can be thought of as a very long string which at low temperatures is tightly coiled into a small ball and floats suspended in car engine oil with little effect on the viscosity of the oil. At high temperatures this ball expands and uncoils and encloses a much larger volume of oil within its structure. This effectively prevents engine oil from thinning because of the greater volume taken up by the expanded balls of VI improver molecules.

At cold temperatures the polymers are coiled up and allow the oil to flow as the low number indicate (like in 5W-30). As engine oil warms up the polymers begin to unwind into long chains and prevent the oil from thinning. Thus, a 5W base oil doesn't thin more than a 30 weight would when it is hot. This is like using two different oils, one for winter and one for summer in one. In the winter base your choice must be the lowest temperature you expect, and in the summer base the highest. The narrowest span with a more heavy base is better if your temperature zone allows, cause a wide span means more polymers, and more polymers are not good for your engine.

Choosing Viscosity Grade

The most known car engine oil viscosity classifications are as follows:

SAE (Society of Automotive Engineers) Classification:

The SAE classifications characterizes the viscosity at low and high temperature of the lubricant. The grade is given by two numbers separated by the letter W (meaning winter).

The first number, followed by W represents the low temperature viscosity, 5W, 10W, 15W... the lower this number, the more fluid at low temperature the oils, the easier the engine starts.

The second number gives the high temperature viscosity: 30, 40, 50. The higher this number, the more viscous the oil when hot.

Below are some examples on choosing car engine oil by expected outdoor temperature:

If lowest expected outdoor temperature is        Typical SAE viscosity grades for passenger cars
0 degrees Celsius (32F)................................................ 5W30 10W30 10W40 20W50
-18 C (0F)...................................................................... 5W30 10W30 10W40
below -18 C................................................................... 5W30

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