In cold climate why oil tank is equipped with oil heaters? Explain.

Oil heaters:  When hydraulic circuit works in cold climate, then oil is solidifies below 5 o C temperature.  To liquefy the oil electrical heater or thermostatic heater are equipped with oil tank.  This increase the operating and maintenance cost of hydraulic system  The cost of hydraulic system is higher.  It heats the oil so that its viscosity increases and it can flow in the system smoothly.  Thermostatically controlled oil heater commonly used