The recommended oil for Vitz (used in Japan) is 0W20.
Since we are using this vehicle in Pakistan, hence due to environmental differences (temp. etc.) the value of viscosity needs to be re-evaluated.
With increase in temperature, higher viscosity is recommended. Hence, with low mileage in Pakistan 5W30 viscosity grade oil is ideal for year round. But, if mileage is above 150,000 KM, a thicker oil can be used like 5W40.
High mileage engines have more clearances between moving parts due to wear and tear and a thicker oil is recommended to compensate for high mileage engines.