umm.. in my modest opinion, (i DO NOT consider myself an expert)..
lets take an example of 5w30 to explain.
5w = SAE 5 (viscosity level, which is quite lower) when the oil was tested in cold temperatures.
30 = SAE 30 (viscosity level, when the engine was tested working at 100 degree Celsius).
Now we all know that weather is extreme in summers in Lahore and temperature hits 48-50 degrees. Imagine the temperature of the engine.. (good cars show that precisely on the gauge). With the air conditioning unit working, and running around in traffic, its mostly either 100 degrees or above..
So we need engine oil that keeps its formation at higher temperatures, i'd say 10w40 is the best bet.. since the oil would loose its viscosity somewhat when operating at high temperatures (even if it says 5w30 i believe it'll come down to plus minus 25 in summer with ac, thats what i believe).. now if the oil viscosity gets lower than required for the engine, it will not lubricate the engine parts as it should.. resulting in damage to the engine.. But at the same time since the temperature isn't really that much and engine warms up pretty quickly, we do not need a much lower viscosity for cold start ups in summers.. like 5w may be unnecessary.. 5w would be required in much colder environment where the engine would take time to heat up to normal.. and in the mean while we need low viscosity of the oil (less thickness) so the oil can run through all parts of engine and do the required lubrication..
Now the opposite situation.. Cold start in winters.. Thing is, at morning startup.. maybe not much of an issue in summers, in winters though when the engine would take time to heat up to normal temperatures, we need less viscosity to lubricate the engine in the mean while (hence multi grading oil was invented by adding additives to a single grade oil).. Now in lahore, maybe.. just mayyybe in winters i may need 5w.. i'd be happy with 10w on a regular car..
Now if you do not own a regular car.. like i had 2zz in indus corolla previously.. it has lifts (hot cams), which if activated require much more engine oil circulation.. and require thin oil.. for that i used 5w30.. i tried to find 5w20 as well, but wasn't satisfied with it (packing seemed old, i was worried it may be expired etc) so didnt get it.