The best viscosity for your New Cultus's engine is 0w-20, which is also recommended in the user manual.
Secondly, 0w-20 Engine oil is fully synthetic whereas other lower grade oils such as 5w-30 and 10w-40 are semi-synthetic or mineral based oils.
The lower economy given by your car was also due to higher viscosity oil being put in it, 0w-20 is a thinner oil and stays thin even at higher temperatures so the engine does not have to work more to give out power, whereas when we use higher viscosity oils then the engine has to work more and eventually gives lesser fuel economy.
I would recommend to not change viscosity of engine oil, as these are old concepts that more mileage engines should have higher viscosity engine oils. New engine technologies require thin oils and if you keep on using fully synthetic oils then your engine life will be much longer.
Hence, you should just keep on using 0w-20 engine oil as it is the best recommended oil for Cultus's engine.
Let me know, if any further information is required on this.