2001 Highlander - Oil Burning

I have recently bought a 2001 highlander with 50K miles. It appears to be consuming oil, particularly during high speed driving, i.E. Highway driving. Checked the PVC valve and apprears to be OK. Air filter is also good. No problems while running. No signs of any leakage. Can't see any blue smoke in tail pipe. Appears to vibrate a bit when idling, but great in Nuetral. Also, mileage is only about 17miles to a gallon. What else can I check before taking it to Toyota? Is this a known problem and will it cost me an arm and a leg? Please help !