The main difference between the grades of oil mentioned here is that the 5w30 is used in places where the ambient running temperature is below freezing point... the high cost of the oil is because it needs to be at a good viscosity at extremely low temperatures and at high temperatures as well...
In most of Pakistan we do not have extremely low temperature therefore I only use 20w-50 oil... you can use cheapest oil but only use original oil filter even PSO carinet is a good oil.
The problem regarding poor performance in your car in my view might be due to a clogged exhaust/catalytic converter. The grade of fuel used in Pakistan is not upto mark and contains many additives which produce a high level of carbon in the exhaust. This carbon clogs the Catalytic Converter due to which, fuel efficiency is reduced, engine becomes harder to start etc.
I hope this solves your problem