Let me begin by saying that i always use the 5w30 from shell but i have a specific reason for that which i shall explain later. Having said that, majority of all our vehicles in rwp, isb, lhr, khi, pesh etc should be using a 10w40. To understand it you need to know what the value 30 or 40 or 50 actually mean? they are a grading of viscosity but what do these figures actually mean? Figure that out and you'll understand my recommendation better.
As far as mineral vs synthetic is concerned, there is no true synthetic (PAO) available in retail in Pakistan and maybe the whole region! all oils marketed as synthetic are actually grade III which is synthetic but with mineral base. True synthetic is grade IV and above. They are hardly available as retail even in western countries. Mostly they are sold in barrels to official dealerships of high performance vehicles only.