U don't need to change anything at 40k kms. Is the mileage calculated tank to tank or its what your meter is showing? Or it's the later, then its a mileage issue for sure. Actually, traffic condition impacts a lot and Karachi has one of the worst traffic situations.
Change the fuel station and drive car with heavy foot for fuel days at hihj rpm...this should clear some clutter from catalytic converter..if not too much clogged which is unlikely at 40k kms. Are u getting oil from dealership or from open market..since there is more fake oil available nin market than original.