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Why can oil that is too thick cause engine parts to overheat?
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lubrication
Motor oil, a lubricant for automobile engines.
In an engine, oil serves the dual purpose of reducing friction and cooling components by carrying away generated heat.12
If the oil is too thick, its viscosity is too high to allow for proper circulation through the engine's internal passages and channels. Because the oil cannot flow through these parts at the required rate, it fails to effectively dissipate heat from the components it is intended to cool. Consequently, the restricted flow leads to overheating.1
To ensure optimal performance, the viscosity of the oil must be carefully matched to the specific design and cooling requirements of the engine.1