MONOGRADE ENGINE OILS usually fall into two main categories. The grades with a w after them (SAE 5w, 10w, 15w and 20w) are only suitable for use in winter time because they are generally thin oils which are good for use in winter or for cold starting.
The summer time grades (SAE 20, 30, 40 and 50) are more suited for warmer (or summer time) temperatures or for use in an already hot engine.