I drive a Toyota Corolla 1.3 litres petrol engine 2010 model. Have 115

I drive a Toyota Corolla 1.3 litres petrol engine 2010 model. Have 115000 kilometres on it. I faced a problem a while back. The head gasket leaked I had to get it repaired. I cannot seem to manage a mileage of over 13 kilometres/litre. Also the car is under powered. I was wondering could it be the oil that i am using? Should I be using a different oil? I am currently using Toyota Petron 5w-20 for it.

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