My first thought was that the piston rings are worn and letting the oil past them from the sump into the combustion chamber. If this is the case you would see blue smoke from the exhaust. As its only done 27k, has it been standing for a long period as the piston rings can stick in the piston, giving the same symptoms as worn rings. I think you would notice any obvious oil leaks from gaskets etc but are you using the correct grade of oil? Try using a thicker grade to see if it helps. Other than this, I am not too sure.