Starts white smoke oil coming out of exhaust?

My car Vauxhall vectra 2007 diesel after driving for about 10 minutes starts to smoke white smoke too much and oil coming out of exhaust and seems to lose power. Somebody suggested could be turbo. I am not sure because sometimes after driving another 20 minutes it stops?

Please help I changed plugs but doesn't help same problem :-) good site for info btw