Oils are graded according to their viscosity by a series of Society of Automotive Engineers (SAE) numbers. The viscosity of the oil increases progressively with the SAE number. An SAE 4 oil would be very light (low viscosity) and SAE 90 oil would be very heavy (high viscosity). The viscosity of the oil used in internal-combustion engines ranges from SAE 5 (arctic use) to SAE 60 (desert use). It should be noted that the SAE number of the oil has nothing to do with the quality of the oil.

The viscosity number of the oil is determined by heating the oil to a predetermined temperature and allowing it to flow through a precisely sized orifice while measuring the rate of flow. The faster an oil flows, the lower the viscosity. The testing device is called a viscosimeter. The viscosity of the oil is printed on top of the oil can. Oil viscosity is written SAE 10, SAE 20, SAE 30, and so on. The letter W will follow

Figure 6-19. - Sources of oil contamination.

any oil that meets SAE low-temperature requirements. An example would be SAE 10W.

Multi-viscosity oil or multi-weight oil has the operating characteristics of a thin, light oil when cold and a thicker, heavy oil when hot. A multi-weight oil is numbered SAE 10W-30, 10W-40, 20W-50, and so on. For example, a 10W-30 oil will flow easily (like 10W oil) when starting a cold engine. It will then act as a thicker oil (like 30 weight) when the engine warms to operating temperature. This will make the engine start more easily in cold weather. It will also provide adequate film strength (thickness) when the engine is at full operating temperature.

Normally, you should use the oil viscosity recommended by the manufacturer, However, in a very cold, high mileage, worn engine, higher viscosity may be beneficial. Thicker oil will tend to seal the rings and provide better bearing protection. It may also help cut engine oil consumption and smoking.