I just read the 500i Owner's manual and nothing indicates how long you should hold the oil service light button down. That does NOT mean anything since there are errors and omissions in all the books, service and owner's alike. I am guessing since I do not have experience with the X- 500i but how long do you have to hold the button to shift or clear odometers? Use that time interval and see what happens. The oil service light is triggered by the odometer every 625 miles! Great for first service since new and useless there on. Your bike new? Warranty issue if it is and you have tried everything. I will bet one of the X-500i riders here will have a good answer for you when they finally come home from work and read your post.
Karl