@ Shurjeel,
Friend, there are different types of spark plugs available in the market for each car.
I do not know which company is selling spark plugs in Pakistan but all I could sum up my experience with regard to the spark plugs is such;
I dont think that there would be any special kind of spark plug avialable for Santro which would improve the fuel efficiency of the car.
Indeed, I had learnt that there are some spark plugs with longer thread portion to reach out better inside the combustion chamber for better ignition. Then there are cold and hot spark plugs, suitable for car engines depending upon its life and condition of pistons/rings ( compression ) or condtion of the head ( valve/seat/guides ). These plugs help an old engine to start better or help start some good engine in a very cold climate.
There could be some more technical attributes regarding spark plugs, but I know this enough..