For my car engine, the car manufacturer already suggest/recommended 10w/40 engine oil so i rather stick with the manufacturer suggestion, plus i don't see much reason wasting my money buying more expensive engine oil grade just for the heck of it.   
Besides it's a known fact that using thinner viscosity engine oil on older engines won't really do much good especially when the engine never had done a total overhauled (eg: replacing piston rings etc) due to u may see higher engine oil loss overtime from engine oil seeping into the combustion chamber. 

it's better if the engine is in good condition however using what recommended by manufacture is still recommended. It's mostly viscosity we are talking about and how it flow, from 10w-30 to 5w-30 is good if the engine is in good condition as the oil will flow faster and better at reaching all parts of the engine when it's cold, however 10w30 to 15w30 is it not! recommended at all.

there is a lot of talking, discussing regarding this topic, you need to read more. "Dont pandai pandai"

Weather is not the concern nowadays, the main thing for most car manufacturer nowadays is fuel economy. Thus why you can see newer hybrids used 0w-16. For local Perodua, i believe the newer Axia, Myvi & Bezza also using 0W-20 already, even Toyota Vios also i heard the same. Coz the lower viscocity, the more "watery" the fluid and lighter the car feel and drive, this will help especially driving in small town or city area a lot. But for sure this will need to refer back to the car manufacturer engine design/requirement, for older engine type if they are design to stick with 5W-30 or 20W-50 just stick with it as changing to even lighter viscosity might increase the chances of oil leak or burn out.