I can't answer about break-in and such but did you verify proper adjustment of the lever on the transmission end of the clutch cable. Mine started slipping last fall and turned out a small adjustment here solved my issue. If you have a repair manual it will detail how to make this adjustment.
As you mentioned, incorrect engine oil can cause this as well. I would exhaust all the options before tearing into the case.
2006 V-Star 1100 Classic
Silverado windshield and passing lamps
OE Highway bar with custom mounting brackets
Kuryakyn Dually ISO Highways pegs
Barons Nasty Boy Slip-ons
Kuryakyn led headlight halo