The numbers 20W50 refer to the viscosity of the motor oil, set by the Society of American Engineers. This refers to its fluidity or thickness. The "20W" indicates how quickly the engine will start when cold or in cold weather. The "50" refers to the thickness of the oil when the motor has been running for a while. 20W50 motor oil is relatively viscous and thick. Most modern oils are 10W40 or even 5W40.
20W50 motor oil is suitable for warmer climates, where the higher temperature causes the oil to thin. It is also useful for vehicles subject to hot temperatures and for those used for high-stress activities such as hauling or pulling trailers.
20W50 motor oil is a high viscosity oil that provides a cushioning and protects against metal-to-metal contact. It is also a more effective sealant than thinner oils. Both of these benefits extend engine life.
Depending on the recommendations for the particular vehicle, 20W50 oil is suitable for large or small diesel engines, gasoline engines, motorcycles and aviation vehicles. It is also suitable for both air- and liquid-cooled engines.