GTX may not be appropriate for wet clutches. Use an oil specificly formulated for motorcycles with wet clutches or at least one that meets JASO MA or MA2 certification requirements. Shouldn't cause a trans problem, though. A high quality synthetic usually helps a lot with the shifting. Higher viscosities than recommended are not necessary with an ester-based synthetic oil--the nature of such oils is to have a significantly higher vapor point than dino oils which results in a significntly lower degradation from heat. Also, a high quality synthetic base oil is composed of identical molecules that tend to flow better through the oil passages than dino oil, which has many different types of molecules that tend to stratify by size and type. Layers of molecules against the passage walls then move much slower than those in the middle and tend to insulate the fast flowing oil from the walls. A quality synthetic doesn't suffer from stratification, thus reducing engine temperatures 10 to 25*C.
All cables should be lubed on a regular basis. Read the owners manual for service intervals, procedures, and appropriate lubricant.
It's a really strange thing, is changing the oil the only real "cure"? I'm currently using Castrol GTX 20W-50
