Dayton Daily News

Halderman: Due to many requests for more detailed information on engine oil, this topic will be covered in detail over the next few weeks.

The most important engine oil property is its thickness or viscosity. Viscosity is the resistance to flow. As oil cools, it thickens. As oil heats, it gets thinner. Therefore, its viscosity changes with temperature. The oil must not be too thick at low temperatures to allow the engine to start.

The lowest temperature at which oil will pour is called its pour point. An index of the change in viscosity between the cold and hot extremes is called the viscosity index (VI). All oils with a high viscosity index thin less with heat than do oils with a low viscosity index. To increase the VI of oil, a long-chain polymer, called a VI improver, is added to most oils. When the oil gets hotter, the polymer expands and helps keep the oil from thinning at high temperatures. Synthetic oil has a naturally high VI and therefore VI improvers are not needed.

SAE rating

Engine oils are sold with an SAE (Society of Automotive Engineers) grade number, which indicates the viscosity range into which the oil fits. Oils tested at 212 degrees have a number with no letter following. For example, SAE 30 indicates that the oil has only been checked at 212 degrees. This oil’s viscosity falls within the SAE 30 grade number range when the oil is hot. Oils tested at 0 degree are rated with a number and the letter W, which means winter and indicates that the viscosity was tested at 0 degree, such as SAE 20W. An SAE 5W-30 multigrade oil is one that meets the SAE 5W viscosity specification when cooled to 0 degree and meets the SAE 30 viscosity specification when tested at 212 degrees.

Most vehicle manufacturers recommend the following multi-viscosity engine oils:

SAE 0W-20

SAE 5W-20

SAE 5W-30

SAE 10W-30

An oil with a high viscosity has a higher resistance to flow and is thicker than lower-viscosity oil. A thick oil is not necessarily a good oil and a thin oil is not necessarily a bad oil. Generally, the following items can be considered in the selection of an engine oil within the recommended viscosity range.

Thinner oil

1. Improved cold-engine starting

2. Improved fuel economy

Thicker oil

1. Improved protection at higher temperatures

2. Reduced fuel economy