SMOKE COMES OUT OF THE TAIL PIPE

I HAVE TO CONSTANTLY ADD OIL AND WHEN I DRIVE LONG DISTANCES OR AT A HIGH SPEED, THE NEXT TIME I START THE CAR SMOKE COMES OUT OF THE TAIL PIPE.

I BOUGHT THIS CAR IN 2008. I GET OIL CHANGES REGULARLY AND OTHER REPAIRS AS NEEDED. IN BETWEEN THE OIL CHANGES, HOWEVER, I HAVE TO ADD OIL. IT SEEMS LIKE I CAN ONLY DRIVE ABOUT 2 AND 1/2 HOURS CONTINUALLY BEFORE I HAVE TO ADD MORE. ALSO, IF I DRIVE THAT LONG OR EVEN DRIVE 30 MINUTES AT A HIGH SPEED, WHEN I START MY CAR AGAIN, A CLOUD OF SMOKE COMES OUT OF MY TAILPIPE. WHAT COULD THE PROBLEM BE?

I HAVE ASKED SEVERAL TECHNICIANS ABOUT THE OIL, BUT THEY HAVE NEVER FOUND A LEAK. A FRIEND OF MINE WHO WORKS ON CARS SAYS IT MIGHT BE THAT ONE OF MY SENSORS COULD BE BAD. PLEASE HELP!