after succesfull experince of fully synthetic engine oil in my corolla, yesterday also replaced my city blood by mobil1 fully syn engine oil 5w/50 
would add some more info about fully synthetic engine oils
synthetic oils are man made product and use as a substitute for lubricant refined freom petroleum when optrating in extreem of temprature because in general it provides superior mechanical and checmical properties than thoes found in traditional mineral oils.
The technical advantages of synthetic motor oils include:
Measurably better low- and high-temperature viscosity performance at service temperature extremes
Better chemical & shear stability
Decreased evaporative loss
Resistance to oxidation, thermal breakdown, and oil sludge problems
Extended drain intervals with the environmental benefit of less oil waste.
Improved fuel economy
Better lubrication during extreme cold weather starts
Longer engine life
Superior protection against "ash" and other deposit formation in engine hot spots (in particular in turbochargers and superchargers)
less oil burnoff and reduced chances of damaging oil passageway clogging.
Increased horsepower and torque due to less initial drag on engine 