What is Engine Oil Viscosity Grade number and oil weight W ?

can i use 15-40W rating lubricant for my bike engine? this is the question that puzzled me when i wanted to replace my bike engine oil recently as it completed 3000km after last oil change though before i did not scratched my head much about it but this time some thing made to know about it, this lead to small R&D and i came to know about what viscosity and grade is all about.

When ever you buy a Engine oil you can see two Information on every can of engine oil can 1) API Rating 2) Viscosity grade

-->API(American Petroleum Institute) rating deals with what kind of engine the oil is meant for whether it is for disel or petrol engine and also quality level... for petrol engines API scales from SA(lowest) to SJ(highest) and for disel engines grade scales from CA(lowest) to CG(highest) you can see this grade on almost all engine oil containers.

-->Viscosity grade for example say 0w-30 or 10W-40 tells you the oil thickness where first number tells the oil at normal temperature and second number which is higher is the weight of the oil at higher temperature... but engine needs oil that is thinner at cold start and thicker at run time(when hot) because it increases oil pressure and reduces oil consumption... though oils in retro age was single grades that is straight 30w or 40w that is 30w when hot and 30w when cold but this made engine hard to start at cold weather's so to counter this problem multi viscosity(Multi-weight) oils were invented which was thin at cold temp and heavy at higher temperatures this made possible by adding polymers to oil to have diff weights at diff temperatures.... At cold temperatures, the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would.

So guys next time when your car or bike needs oil change use the oil as recommended by your vehicle manufacturer in the manual for better engine performance