I'm still trying to figure out from all the comments and the owner why everyone thinks the clutch is slipping or engine oil is the problem.
Did the OP say the engine revs up in 5th gear with no forward drive? I haven't really read that in any definitive way.
I've never had a bad clutch in a bike were it wasn't obviously slipping.
Communicating with a novice is a bit hard and it's obvious that the OP is new to moto-mechanicing and would benefit greatly from someone checking it out for him in five mins.
As I mentioned very early on, fuel starvation still seems a likely candidate.
It seems like a reving engine with lack of any additional forward drive would obvious, even to a new rider.
He should try to start the bike off in 4th or 5th gear and see if the engine just stalls out.
If no, the clutch is crap.
