Your Car's Engine Oil needs to be able to Flow during a wide range of temperatures. This is also a consideration where you live and what your Driving style is. Hotter climates need a Thicker Oil than cooler climates. The Engine Oil has to be thin enough for cold starts. But be thick enough to not be too Thin when the engine is hot. One of the Characteristics of Oil is it gets Thinner the Hotter it gets. This requires the use of a Multi-viscosity Oil.
Multi-viscosity oils are a blend of low-viscosity base oil. Then they add Viscosity Index Improvers. So you take a SAE 15W Oil and add the correct proportion of VI Improvers. The Engine Oil will perform like 15W at low Temperatures and SAE 40W at High Temperatures. True Multi-viscosity Oils will have 120 or more Viscosity Index Improvers. Most Modern Engines will need to use 0w20, 5w20 or 5w30. Please check your Owners Manual for OEM recommended oil uses for your area and driving style. You can also look at the Oil Fill Cap on your Engine it often has the Oil Viscosity printed on it.