Dude I had the same problem, Most likely to be oil (i know every1 said same thing) but the vtec runs of oil pressure (why the S didnt come with a oil press gauge i dont know!!) i was driving and had very laggy vtec and when i pulled to a stop i would hear tick tick i went and did a full service and only dropped 550ml of oil from the motor!.

I was talking to a shop who does engine rebuilds nd he told me that f20c i one of the best 2lt motors out but he did say that the AP1 f20c makes more power but drinks oil compared to the ap2 f20c. he said it was cause of the sleaving they use in the block -

hope this helps and check your oil.

oil and filter = $80 or there bouts
Rebuild = $2000+ depending on who you know.