Kindly allow me to offer my two cents on the subject. there is a very common misconception that oil quality degrades after certain use. well....this misconception is only partially true. in reality, oil molecule be it synthetic or mineral never degrades. it retains its shape and hence its ability to lubricate even after thousands and thousands of kilometers. it is the engine break-down debris and combustion residue that contaminates the oil and significantly affects its ability to lubricate.
in a typical modern vehicle, oil lubricates, collects engine break-down debris and combustion residue then passes through the oil filter, trapping all the harmful stuff there (atleast in theory). most studies have shown that engine wear is actually caused by these suspended objects (in oil) and the most harm is caused by the engine break-down debris with particle size of one nanometer or less. why its the most harmful? well....one, it is metal and acts as mini-grinders. second, they are so small that they cannot be trapped by a typical oil filter.
so...whats the moral of the story? well..... its not the life of oil, its the contamination that necessitates changing of oil. so.... the sooner we change the oil, the more we will prolong the life of the engine.
Now lets apply this to motorcycles:
in motorcycles the oil filters are hardly efficient enough to clean the oil properly. some even don't have oil filters to begin with. meaning there is no mechanism to collect the BAD STUFF and prolong the life of the engine lubricant. secondly, motorcycles engines (single cylinder specially) are run at way higher rpms, meaning elevated temperatures and speedier break-down of engine components, meaning higher quantity of engine break-down debris/particles in the lubricant. finally, the oil is used to lubricate gearbox as well which adds more strain to the stressed lubricant.
having said all that, if the recommended drain interval (for a given oil) in a vehicle is say 4,000 kilometers then for a single cylinder motorcycle it should not be more than 1000 kilometers. if the motorcycle is provided with an oil filter then it should not be stretched any more than 1500 kilometers.
Remember: its not the quality of the oil that matters. its the contamination. synthetics and higher quality mineral oil just have better viscosity indexes and can lubricate through a wide range of operating temperatures. secondly, they also resist (oil) oxidation therefore reduce the quantity of combustion reside in the lubricant but offer no protection against engine break-down debris. which is a fact of life as far as engines are concerned.
Moral of the Story:
its pointless to use super expensive super synthetic engine oils for single cylinder motorcycles no matter how many gazillion kilometers of drain interval they offer. just use any adequate quality engine oil. in my humble opinion, just use any medium range (350 to 500) rupee range lubricant and change after every 1000 kilometers or 1500km if your bike has an oil filter. thats that best you can do.