Seems strange, I would describe my 2012 as a medium weight clutch, far from heavy.
Assuming the clutch itself works perfectly otherwise when riding, you shouldn't need a new cable unless there is evidence of any fraying or outercable damage (tight bends, pinched or bare sections). Whilst not wanting to preach to the converted when you say you oiled the cable did you actually lube it thoroughly, using a pressure oiler. It's easy to assume a quick squirt of oil will do the job but it is important to wait until the lubricant finds it's way right down to the actuating mechanism LH of engine. Alternatively tape a small funnel over the cable end, top it with oil & leave overnight. Then it's worth removing the lever pivot bolt, clean it up & grease the pivot & cable nipple area thoroughly before replacing. Don't forget the pivot by the LH of engine too. It should be easy to check, when disconnected & lubed up, that the inner cable runs freely within the outer sheath.
If you have done all that & the clutch remains heavy then you just have to wonder whether there are issues within - could say a previous owner have fitted heavy weight/competion clutch springs/plates. Is there an issue with the plates moving freely on the clutch hub as spring pressure is taken up.
Good luck sorting the issue. Keep us posted...................... KEN
