two things,
1 ) don't worry, change the gear oil with the original Honda oil for this model of City. for manual car, people don't change gear oil for 10+ years and it does not hurt the gearbox, so it's okay to change gear oil now.
2) try mileage test on 100 see how much do you get then and use the same petrol pump that you previously did.
most 1300 cars reach 120 at roughly around 3100 rpm .. i don't know why Honda has used this gearbox with shorter gear ratios ..
khair, clean air filter, fill required tires air pressure and test again..