I have lost count of the number of times a customer comes in asking for fully-synthetic oil for a regular motorcycle. Manufacturers specify, for arguments sake, a 10w40 semi-synthetic oil from their recommended oil supplier, be it Honda with Castrol or Yamaha with Yamalube, etc... It'll have a specification dictated by country, pollutants and additives in oils, designed specifically for that motorcycle. So why would you make the rash decision, and more expensive option to change to fully-synthetic after 15,000 kilometers running faultlessly?
People do some strange things, Their friend on a online forum syas, “I've got this in my motorcycle,” and they haven't actually thought that this motorcycle may be a KTM, for example, and requires a certain oil. Most manufacturers recommend a 10w40 to cope with a wide parameter of working temperatures and abuse, and work with this oil consistently as a base. Run-of-the-mill Honda Fireblades and Suzuki GSX-R's go on and on, being ridden to the moon and back – or at least ridden round the world as proven by some extremest on motorcycle TV shows. Do they run fancy fully-synthetic 0w50 oil for this around the world trips? No.
So they go and change this oil anyway, and that's when problems start; the motorcycle isn't as smooth as it was, perhaps there's a bit of clutch judder. There is a common myth about putting fully-synthetic oil after running semi-synthetic oil that the clutch slips, but that's rubbish and more to do with cars and automatic transmission oil types that have friction modifiers as additives.