A motor oil chart is a chart that displays and ranks the various grades of motor oil viscosity. Viscosity is determined by measuring the time it takes for a standard amount of oil to flow through a standard orifice, at standard temperatures.
The Society of Automotive Engineers ranks single-grade motor oil using a numerical system, with zero being the lowest viscosity and 60 being the highest viscosity. The grades between zero and 30 increase in intervals of five, but the intervals increase to 10 from 30 upwards. The rankings are written out with a "SAE" prefix and then the number grade, but sometimes a "W" is connected to the number; the "W" means the oil was tested at colder temperatures and should be used as a winter oil to help the car start faster. Grades 0, 5, 10, 15 and 25 are also accompanied by a "W", but grade 20 is available in both winter and nonwinter varieties.
Single grade motor oils become multi-grade motor oils when polymer additives called viscosity index improvers are added. The additives reduce the difference between an oil's viscosity at lower temperatures and its viscosity at higher temperatures, allowing one type of oil to be used all year.