Chemical Process
Viscosity index improver (like polystrene or polyisobutylene) is added to lubricant to

Increase the variation in its viscosity with temperature
Increase its viscosity
Reduce its viscosity
Reduce the variation in its viscosity with temperature

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Chemical Process
Mineral oils (e.g. petroleum oils) are preferred over fatty oils (e.g. mustard oil, ghee, tallow, palm oil, olive oil etc.) as a lubricant due to its

Poor oxidation stability and high gum forming tendency
All of these
Hydrolysis tendency in presence of water
Greater tendency of decomposition at elevated temperature

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