Mubarik ho bro. Firstly these 1000cc vitz/belta are very good to drive and are comfortable as hell, one shouldn't call them bad just because they are sluggish.
For oil go for 5w-40 mobilone semi-synthetic or liquimoly 5w-40, both are very good, they are long lasting and make the ride smooth plus they also increase fuel efficiency plus go to filter house and get all the filters changed from AC to air filter
For tuning, go to toyota airport, and waha call the foreman Basit, he has ample knowledge of vitz or belta 1000cc, tell him to clean the throttle body, check or clean spark plugs and computer laga k race etc set kerde.
Sent from my SM-N920C using Tapatalk