Rear wheel turning in neutral ?

[slappy]

If it is creeping in gear then definitely clutch  drag. If the Yamaha dealer changed the oil and it had fully synth in before and now it doesn't  I would not have thought that would cause the clutch to drag but could be wrong on that.
Try backing of the clutch adjustment to see if that helps.
I would take it back to whoever serviced it and tell them you are not happy, after a service engine should be running smoothly and clutch working correctly.