This is true that 0w20 will give you the best mileage, but for old engines (above 100,000 km) it's better to use a thicker one like 5w20 or 5w30.
Also, in its difficult to find 0w20 in Pakistan.
5w20 ( Zic A+) and 0w30 (Zic 0) are the closest ones available in Pakistan for best fuel economy. It is to be noted here that 90% of engine wear and tear thanks place during cold start ups till the engines comes to the normal operating temperature. Hence a thin oil (0w20 etc.) is required to keep the engine well lubricated during cold start ups.