the oil become black or dirty because of carbon deposits from petrol. if you look at an engine that has only been run on LPG the oil is gold like the day it was put in, just the internal properties are degraded.

while black oil does not nesscarily dictate the quailty of oil, it usally has been in the eginen long enough to have the internal qualities degraded. if you look and diesels the oil become black very fast, oil is still good, just black.

when you change your oil is up to you. if you oil becomes dirty because of "dirt" in the air you have mayjor engine issues. point of this? if you ride it hard, change it more often, if you ride it on the road you can go long between changes.

id say 500km too 2500km

i do mine every 1500-2000 and a filter every 2 or 3 changes

good luck