excessive smoke on start-up

Vehicle Information: Year: 1997; Engine (Size, Modifications): twin turbo; Total Mileage: 15,100;

i just bought this vehicle after searching for a year for a low mileage 993 turbo. This car had 14,900 miles on it when i bought it and checked out fine. I immediately took it to my local dealer for the 15K service. the car then sat in my garage for 6 days, and when i started it up, thick white smoke filled my neighborhood and lasted for 25 minutes. The next day i started it after it had been sitting for 12 hours, and the smoke returned but lasted only 7 minutes.the dealer thinks maybe the former owner used a different visocity of mobil 1 then the 15w-50 they put in. I noticed several similar problems dated in 2003 at your site, and am wondering if there is any more "current" thinking or hopefully solution for this issue. thank you.