Depending on the temperature, the viscosity of the oil changes. As the temperature rises, it becomes more liquid. The strength of the oil film deteriorates. Reduced extrusion resistance. When cold, the oil is dense and viscous, fluidity is poor and internal friction increases. During operation, engine oil is required, which would change as little as possible with temperature changes.
With a cold engine, it should be fluid enough not to unduly impede the operation of the starter. And after starting, it should, as quickly as possible, go to the lubrication points.
Viscosity is the internal friction of the oil - which is indicated by the SAE system (Society of Automotive Angineers), such as SAE-30, SAE-10, etc. a higher SAE number indicates a thick oil, a lower SAE number indicates a thin oil. But the viscosity cannot be used to judge the lubricating properties of the oil.
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