Min 5w Max 40 is suitable for all time operating conditions in pakistan...(no need for further clarifications)
Why thicker oils are not good..
thinner motor oil is essential for easy starting, particularly in cold weather, and for proper lubrication once the engine starts. Today's smaller engines have smaller clearances and tighter tolerances between moving parts, and there have been some instances where camshaft damage has occurred because of inadequate lubrication with higher viscosity grades in colder weather. Thinner oils, such as SAE 5W-30, will flow faster than heavier motor oils during start-up and initial engine operation and will help protect the engine from excessive wear. Multigrade oil will also offer the same high temperature protection as single grade motor oil.
Always check your vehicle owner's manual to select the proper viscosity grade based on the expected temperature range...FOr example a 5w40 maybe suitable for a specific engine till 60C but for another engine it may be suitable till 50C etc etc.There are 2 point to consider 1) recommended Viscosity 2)CLimatic temperature..Usually manufacturers give 2 recommendations for oil viscosities..ANd as a rule of thumb..CHECK YOU ENGINE"S OIL CAP and follow the stated viscosity.
The viscosity grade(s) recommended by the vehicle manufacturer depend somewhat on engine design. Engine manufacturers have spent considerable time and expense experimenting with different viscosity grades and have indicated in the owner's manual the grades they feel will best protect the engine at specific temperatures..SO dont mess with the manufacturers..If you got buckloads of money atleast use them wisely.
20w50/10w50 oils offer protection for older engines and prevent excessive wear..They recondition the seal and prevent further wear
Also remember the Pour point and Flash points of oils vary by manufacturer..SO do check them out..two same 10w40 oil of dfferent make may have different PP and Fp.