As the engine heats up and passes its heat to the oil, the oil gets thinner and flows more easily. Single grade oils get too thin when hot for most modern engines which is where multigrade oil comes in. Most modern oils of today are multigrade. By altering the chemical balance of the oil it is prevented from getting too thin when it gets hot. This prevention is described as the viscosity grade. For example "5W/40" is a two-part description. The number before the 'W' is the 'cold' viscosity rating of the oil, and the number after the 'W' is the 'hot' viscosity rating. So a 5W40 oil is one that behaves like a 5-rated single grade oil when cold, but doesn't thin any more than a 40-rated single grade oil when hot. The actual testing is done at 100 °C which is the typical operating temperature of the engine. The lower the 'winter' number (hence the 'W'), the faster the oil will reach all the crucial areas of the engine at cold start.
Performance engines will typically run low viscosity oils, such as 0W/40 or 5W/40, whereas a non-performance engine will run a higher viscosity, such as 10W/40.
Viscosity grade is only part of identifying the correct oil. Specifications must also be taken into consideration. A number of specifications have been defined by the various manufacturers, such as MB 229.51 and VW502.00. The specifications ensure that the oil lives up to the requirements needed to run the engine optimally. These requirements include, cooling efficiency, oil degradation, oil additives and more.
Oil degradation happens through:
The manufacturer will specify the oil change intervals based on the oil specification. A long-life specification may allow for intervals of 30.000km and multiple years, whereas a non-longlife oil may need changing after 1 year or 10.000km.