When the engine warms up, the oil thins out. If it becomes too hot and thin, the oil film can break down and part contact can result. It is important that the oil be thin enough for starting. Ii must also be, thick enough to maintain lubrication when hot.
Multi-viscosity oil, or multi-weight oil will exhibit operating characteristics of thin, light oil when cold and a thicker, heavy oil when hot. A multi-weight oil car; be numbered SAE 1OW-30, 10W-40, 20W-50 low 20W-50, etc.
For example, a Iow-30 weight oil will flow easily (like a low 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.
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