Ever heard a thing "Wearing out of Mechanical parts", well it actually exists. Owners manual is only good for first few thousand miles then you have to shift to a thing called "common sense". If engine reduces oils, only a fool will keep on using thinner oils recommended in manuals. Secondly first number in oil viscosity is only significant in extreme cold conditions, but still thinner will always be better no matter where you are. You need to look at the second number, that is viscosity at operating temperature so basically,for example in lahore 0w30, 5w30, 10w30,15w30 or 20w30 will all behave exactly the same after a minute or so.