Dear Jannathulla, no reasons ( do you intend methods?) other than temperature exist for reducing oils viscosity; viscosity is an intrinsic chemical-physical property of a fluid.  Gianluca

Seems your question is very confused.  Increasing temperate is not a reason for reducing viscosity - typically viscosity reduces as temperature is increased,  You may wish to reduce viscosity of an oil so that it flows more easily, so could be pumped more easily, remove air bubbles more easily etc.  Other routes to reduced viscosity are to mix with a less viscous material that is compatible with the material etc.

working lif and impurities are considered the main causes of viscosity reduction after temperature.

The use of methods instead of increasing the temperature is due to the thermal decomposition of the oil, which  affects the properties and composition of the fuel

Viscosity is an intrinsic property of oil which may also depend on the degree of unsaturation.

Mixing varities of oil brands can reduce the viscosity. In engines if tthehere is a leak where oil is mixed with gasoline or diesel.