Motor oil is used for lubricating the internal parts of internal combustion engines in a wide variety of applications. It helps control temperature and allows moving parts to function without generating excessive friction and wear. Some oils can even act as detergents to help remove combustion deposits that may otherwise degrade engine performance. Engine oils are classified into grades by their thickness, or viscosity. Changing your oil regularly with the grade recommended by the manufacturer is an essential part of proper engine maintenance.
Oils are grouped into grades according to viscosity guidelines set forth by the Society of Automotive Engineers, or SAE. The viscosity of oil refers to its internal resistance to deformation, or resistance to flow. Viscosity is measured in multiples of 5 from 0 to 60, from low viscosity to high: 10W, 15W, 20W, etc. The W attached to some of these ratings stands for winter, because SAE ratings take into account the viscosity of engine oil in cold-start scenarios.
Single vs. Multi-Grade Oil
Because most vehicles are operated under different conditions, a single type of oil may not offer the ideal viscosity characteristics to protect vital engine parts at different temperature ranges. Engineers have addressed this problem by essentially combining multiple grades of oil into a single formulation. Special polymer additives are employed to give the oil different viscosity characteristics, and these oils are typically denoted with two figures: 10W30, 5W30, etc. The first number refers to the viscosity when cold, and the second number refers to the viscosity when hot.
Multi-grade oils help ensure that engines are fully protected from friction and wear across a wide range of temperatures. They are rated using the same viscosity scale as single weight oils: 5W30 would be a "thinner" oil than 10W30 at cold temperatures, for example, and would typically warm up to the same viscosity as the engine heats up. Using 5W30 rather than 10W30 would thus have the benefit of increased lubrication in cold-start conditions, and it would be the preferred oil for those living in colder climates.
Most newer vehicles produced in the United States are equipped with engines formulated for relatively thinner oils than were used in the past. Thus, a new car in 2011 might specify 5W20 instead of 10W30. This difference is primarily due to improvements in machining and materials technology, which allows engines to be assembled with tighter bearing clearances that resist wear much better than in the past. Using "thinner" oils allows for better fuel economy as well, which is an important consideration in an era of steadily increasing fuel prices.