Brother someone has misguided u big time.
Synthetic will NOT improve the engine power or fuel economy by even .00001%.
I have used all synthetics like Mobil 1 and even Amsoil in my new shape Civic 2012.
Instead if you will use synthetic in a 5 years old car you will end up leaking all its engine seals and have to purchase the engine kit with all seals.
Use simple Shell Helix HX-3, HX-5, Castrol GTX in your Santro and change it on time.
Synthetic is a waste of money for your car, believe me.