I'd go with the others here and check the outside stuff first.
Take both ends of the cable off and pull the inner back and forth to see if it's dragging at all and sort that if it is, try silicon grease in the worm mech as this will not usually change and get gummy over time and use.
Ride it with excess slack in the cable to make sure that is definitely not holding the clutch open at all.
The different oils mentioned earlier (don't think it's this as you seem to have an appropriate spec in there) as I understand it are to be avoided if they contain "mollybdenum" (maybe not the correct spelling) as it's this friction modifyer that is considered accumilative in clutch materials, making them slip and hard to get rid of with a change back to other oils. I don't believe that "synthetic" as a general term is to blame.
