The viscosity of motor oil is measured in SAE (Society of Automotive Engineers) grades, with lower numbers indicating thinner oil and higher numbers indicating thicker oil. The thicker the oil, the more resistance it provides to flow.
The engine in the Yamaha SR 250 Classic requires an oil that is thick enough to provide adequate protection against wear and tear, but thin enough to flow easily through the engine's oil passages. The manufacturer recommends using a motor oil with an SAE grade of 20W-50.
The "20" in 20W-50 indicates the oil's viscosity at cold temperatures, while the "50" indicates the oil's viscosity at hot temperatures. The "W" stands for "winter," and indicates that the oil is suitable for use in cold weather conditions.
