...an interesting post!
My first 660Z was serviced at the Yamaha dealer where I bought the bike (this is the 600mile running in service). When I checked the bill I noticed I had been charged over �30 just for the oil; turns out they put an expensive fully synthetic oil in...at 600mile!!! I wasn't happy but what could I do, the oil was in the bike. Saying that there were no issues with clutch slipping.

Personally I'd only put a semi-synthetic in a 660Z.
I have an XR400 and I know they are very sensitive regarding oil types/clutch slip issues, and water cooling aside, a similar engine to the 660Z.