Nothing to do with the clutch issue, which I would like to see a solution to as mine is "abrupt" too...
The oil is the same through out, but from what I can figure...
Gearbox and clutch are lubed by oil thrown off the main gear on the clutch, this is picked up off the bottom of the engine same as the 2T gearbox oil and drained via the same screw.
Piston and crank are lubed via splash from oil captured in a small reservoir around the base of the crank, separate drain for this.
Top end roller bearings for cams, roller lifters etc are fed via oil picked up by the cam chain from a reservoir around the crank sprocket, with its own drain.
There is a one way reed type valve that is used to transfer oil from somewhere to somewhere else, but I can't figure that out from parts book or anything.
The oil filter is a small metal affair with about a 2mm hole in both ends and I don't see that it really does much.
I am running semi synthetic 20W-50 but it did not help.
I am thinking of tyring something like Automotive Mobil Rally one next.