The oil grade should be according to your owners manual. Have a look, I think it would be best to use 5W30 grade oil for best fuel economy.
Keep in mind the quality of oil you use does matter and if the mileage is not high then shift to a fully synthetic grade oil.
Other than that, pak suzuki is entirely useless for resolving most problems especially those related to the AC. If you are in Karachi I can suggest an AC repair shop that can resolve your problem of cooling at idle.