Oil on spark plug electrodes

[His Dudeness]

Are you 100% certain it was oil on the plugs and how is the bike running? What mileage is on the engine? How much oil is it using? They can get black from carbon and wet from petrol. If it is oil you should do a compression test as Red98 said. That'll tell you what condition the cylinders are in. Oil can only get into the cylinder two ways either past the piston rings or past the valves. If you do a compression test and it's low, repeat the test again with a little oil added to the cylinder and if the compression goes up you'll know it's the rings that are the problem