You are already feeding the right viscos oil that your car need.
Yes, 10/40 would be exact viscosity.
If satisfied with ZIC A, then keep it using. You're already getting 16~18 km/l, its pretty good.
So, I'll suggest DO NOT CHANGE & keep using it.
However, if you want to give a try, then choose little high grade synthetic oils like
PTT Performa 5/40. Price is Rs. 800/liter. Change it after 5~6 K.
Liqui Moly's 10/40. Price is Rs. 1000/liter. You can easily consume it till 5~6 K.
I'll not recommend to use LM 5/40 Synthoil because its expensive and also your car engine is bit smaller for it.
But if you try PTT Performa 5/40 and found good results (better than ZIC A), then you can try for LM's Synthoil for the next time.