Hi Rateeb! Oil Viscosity is actually a very important factor when it comes upon which engine oil to choose. In simple words, Oil Viscosity defines the thickness of the liquid. For example, Honey has the highest viscosity grade and water has the lowest viscosity grade. So when you choose the engine oil for you car, make sure that the viscosity is right for your car as per owners manual. Every car recommended engine oil specification is written in owners manual, wheather fully synthetic or semi synthetic oil is needed like 0w40, 5w40, 10w50 etc.