when i purchase my car ,0w20 was already inside car's engine ,drive was very smooth, fuel consumption was also great ,because it is for new engines ,Very thin oil ,In japan they use it for cold weather ,for easy start up ,even at side door of my cars; 3 stickers are dispatched of said oil veloscity with duration of change ,then i tried to search and found ,it suites only japan temp ,in our country ,where temp get rise in summer ,It wouldn't fill engine requirement .therefore i changed it to 5w30 in winter ,and for now 10w30 ,
P.S this is from my low knowledge .