as temperature in Islamabad can go below 0, 10w-30 will be better for you. 20w50 is suitable from 0C + temperatures.
for a perfect engine, i will choose 5w-30 for top performance and economy. for a normal engine 10w-40 will work good.
i tried total/caltex 20w-50 in my cuore. there is not any starting problem in winter. neither is any notable performance difference. but when i traveled on long i noted that economy had gone much bad. 15km/litre on long. but thick oil is good for bit weak or oil burning engines. with a thicker oil, u can expect longer engine life on expanse of economy. performance difference is not notable.
