heavy oil smoke

2004 Focus 2.3, auto ,186,543 miles. At highway speed (55-70 )heavy blue oil smoke from tail pipe for 10-30 seconds,it will stop, then in a minute or two it will do it again. when oil level gets 2 qts. low it won't smoke any more. I replaced the PCV valve with a part from Advance Auto.Still same amount of smoke. Any one know what it could be ? Thanks Denny