There are many threads on this forum explaining what to do after buying a used car , you should’ve searched.
After buying a used car , it’s recommended to change all the fluids and filters ;
Engine oil
Transmission oil
Brake fluid etc
You should also get a routined maintenance done from a reputable workshop. I would recommend you to use original air and oil filters from the dealership maybe which are a bit expensive but are beneficial in the long term.
I own a swift too and have been using original oil and air filters and the car hasn’t given me below 12 with a heavy foot on ac .
Use 10w40max , higher viscosity than that will suffocate the engine.
