Most manufacturers suggest you use a semi synthetic oil in your bike. Lets use a GSXR600 for example, a 10w40 semi changed every 3500-4000 miles. Why do oil manufacturers sell fully synthetic? If you use your bike for track days then a fully synthetic oil is a very good idea and it will be able to cope with the high temperatures and continue to offer the anti wear properties even at competition level performance. Another good use of fully synthetic oil is when you dont use you bike very much, it sticks to metal much better than a semi oil and will help prevent engine wear on start up if you have not used your bike for a few weeks. If you use your bike regularly on the road and change the oil at regular service intervals then a semi sythetic oil is perfect.