Duration of changing Engine Oil in Bikes:
Will split this into two cases.
Case 1: A Normal Rider (Riding minimum distance to within 60 - 100 Kms per
Case 2: A Long Rider (Riding more than 300 Kms per Day)
For Case 1: Person needs to change it after certain kms say 2500 - 3000 kms.
For Case 2: Person needs to ensure the property of the oil remains good and if in case, it low as engine eats a very small amount of oil over greater distance. If so, can top up and retain the oil level in the bikes.
However checking oil levels in the bikes is mandate after 2000 kms. for both the cases, thereby ensuring its in good condition and not in pitch black color if so, engine oil has to be changed.
Engine Oil's impact over mileage:
If the engine oil quality and condition is good and it will promote the mileage of the vehicle to a greater impact.
On the other hand, over a period of time, as the engine oil gets polluted (by its operation of circulating to all parts of the engine), contaminated by the pollutants (minuscule) particles obtained via wear and tear of the engine during its operation (i.e. on riding).
As far as mileage is concerned. Always ride your vehicle (Bike) on its top gear, as far as the gear ratio of the vehicle is concerned, it fetches you the best mileage. Since the gear ratio will be 1:1. By this, the highest mileage of the vehicle is achieved.
In addition to that, engine oil flush to be done after every 50,000 kms in a vehicle to ensure the engine health is good and clean.
Adding the engine oil additive to the engine oil also, aids the engine oil for better and smooth performance of the engine thereby enhancing the better mileage of your vehicle to a greater extent.
That's why all the vehicles gives the better mileage on highways than driving in a city or a congested or a within a highly populated place, Since, you need to do a lots of gear transitions over a short distance.
Some other parameters which may influences or promotes the mileage in bike are:
Riding Style (Riding at a consistent speed)
Engine Oil Quality
Smooth Transmission (Chains are cleaned and lubricated well)
Lesser the Power (Less CC) of the vehicle more the mileage.
Ensuring the Tyre pressure are at recommended level (Filling Nitrogen Gas
Cleaned Air Filter and Carburetor
Climate or Environmental temperature (To some extent)
The higher the quality of the engine oil, higher the mileage. In other words, if engine oil seems to be good over a period of time then it will enhance your mileage.
Hope this information helps. And the answer is relevant to the question requested to some extent.