I think the figures stated in the owners manual state roughly 1 litre per 600 miles if i remember rightly. Alot, i know but going off your figues of 6000 miles and oil is halfway down dipstick, i'm guessing between min and max, then its used no more than a litre i'd say. As for the Knock sensor, that price seems a little steep. How much was the part? There's a possibility its been damaged when changed over to new engine. Could you get one from a breakers and try that?