My guess is that your oil is getting to thick with the cold. Looking in the news, Europe is experiencing a prety harsh winter presently.
I'm guessing that you are European as that vehicle is not available in North America.
You should use some high quality multigrade oil, like some 10W40 or even 5W40.
I live in Québec, and that kind of oil is always used.
Another thing to look for would be to install some battery heater, as your block heater can't warm up the battery.
Kuälinar
February 2012