Yes, feels, not sounds like luckily It's a really strange thing, is changing the oil the only real "cure"? I'm currently using Castrol GTX 20W-50
It may not be internal. TW transmisisons are a bit temperamental, but here's a couple things you might want to check if you haven't already.
1) Chain tension. Too tight or too loose will cause shifting difficulties.
2) Clutch cable return spring not connected. There is a return spring on the clutch arm on the left side of the engine. Many have reported this spring is not over the arm correctly. Follow your clutch cable down to the engine and you'll see what I'm describing. This thread will show a picture of what I'm talking about. http://tw200forum.com/index.php?/topic/431-clutch-slipping-check-spring/
Just a couple idea's for what it's worth.
Wow, thanks for the great explanation I've just bought a 5L of Castrol ha ha, gonna try sell it to a friend with a car...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.