Semi-Synthetic or Fully-Synthetic Engine Oil
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.
In short, if you've got a motorcycle designed to run on semi-synthetic engine oil and you're thinking of running fully-synthetic engine oil, don't bother unless you want to spend money on something you really do not need to, or unless your circumstances change and you're racing or tuning the hell out of the engine… and pushing it to the limits.
Flying Squirrel
From a well know site:
"Semi-synthetics" are oils which are a blend of petroleum oil and no more than 30% synthetic oil. If the manufacturer adds no more than 30% synthetic oil and does not change the additive package, they do not have to re-certify the oil. These days, since everyone has agreed that Group III base oils are "synthetic," I'm not sure "semi-synthetic" means anything at all. The manufacturers love this stuff: it costs about 15% more to make the oil, and they get to charge about double.
Here in Thailand it costs nothing to change the oil, unless you wear a business suit and ride a Harley.:-) Where they will tell you all the stuff has to come from Singapore!
It is the combined gearbox that does most of the damage. If you live near the dealer use regular oil and change it at the required interval. As has been said the older bikes get along fine with regular dino oil.
If you don't live near the dealer and the time and money spent getting there is inconvenient then use fully synthetic which a has a slightly longer replacement interval and change it less often. By synthetic I don't mean Amsoil or Redline or other expensive stuff , just regular Shell, Mobil et al. as long as it is MA rated for motorcycles with wet clutches.