Viscosity characterizes fluidity of oil. Depending on temperature oil tends to fluidity reduction. In process of increase in temperature oil becomes more fluid. It worsens a stseplyaemost of oil and ability to maintain high contact pressures. When cooling oil gets denser; fluidity decreases and the internal friction increases. Because of this property it is necessary to apply engine oil with such viscosity which changes at change of temperature in probably smaller limits.
At start of the cold engine oil should be sufficiently liquid excessively not to overload the engine and that after start oil got into all places where greasing is necessary quicker.
Fluidity of oil is equivalent to its internal friction and SAE 30, SAE 10 is designated by units of SAE (Society of Automotive Engineers), for example. The high indicator of SAE means that oil dense, low characterizes liquid oil. Nevertheless, viscosity of oil completely does not characterize lubricant properties of oil.
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