Oil viscosity is something most folks have heard of, sort of understand but are not real certain about. This is not intended as a technically complete piece on the topic. Rather this is a light overview as the term relates to engine oil.

The Society of Automotive Engineers (SAE) has provided a scale by which oil can be classified. This is the familiar "weight" rating. For instance 30 weight motor oil refers to a certain viscosity. Basically viscosity is a resistance to flow or sort of how thick the oil is.

In reality the SAE numbers refer to results of testing procedures. The test might involve how long it takes a measured quantity of oil to drip through a fixed opening at a set temperature. The temperature used in testing motor oil is 100 degrees Celsius or about 212 degrees Fahrenheit. This is close to the operating temperatures in most engines.

A certain quantity of SAE twenty weight oil would take 5.6 to 9.3 seconds to drip through the opening.   In the SAE rating system the higher the number the thicker the oil. 

Oil is also thicker when it is cold than when it is hot. Engines operate over a wide range of temperatures. For instance an engine may start at 70 degrees Fahrenheit and warm up to 212 degrees. Oil thick enough to protect the engine at 212 degrees may be too thick to flow at 70 degrees. This deprives the engine of needed lubrication when it is cold.

Multi-viscosity oil solves this problem. By adding viscosity improver (VI) to oil the thinning out at temperature is greatly reduced. This means it stays very near the same viscosity over a wide range of temperatures. An example of an SAE multi-viscosity oil is 5W30.

The 5 means the oil can flow like a five-weight oil at low temperature. The W means winter and refers to the suitability for cold weather use. The 30 indicates the oil will still test the same as a thirty weight oil at 100 degrees Celsius.  The oil specified by the manufacturer will give all of the lubrication needed, for the life of the engine. 

Modern engines commonly use 5W30 and 5W20 oil. This provides for good lubrication on startup and full protection at temperature. A popular misconception is that 10W30 oil is thicker than 5W30. At 100 degrees Celsius (operating temperature) they are the same thickness, 30 weight. The problem is on startup, 10W30 cannot flow (lubricate) as fast as 5W30.  Lack of startup lubrication can drastically shorten the life of an engine.  Always use the oil specified by the vehicle manufacturer.

Along with lubrication, oil also cools, cleans and protects the engine from corrosion. On engines with variable valve timing and displacement on demand, it also operates the necessary components.  Many engines also use hydraulic tensioners on timing belts and chains.  Using the improper viscosity can cause several problems. For instance, putting a thicker oil in an engine can increase volatility and damage an expensive catalytic converter.

Oil need not be a complicated subject. Using the oil specified by the maker of the vehicle and changing it on a regular basis will help keep your engine running for years to come.