I recently purchased a 2011 Sportwagen TDI and so far I am thrilled not only with the gas ( I mean diesel ) mileage, but the way the car drives, handles and the interior of the wagen. Being new to diesel, I have heard a lot of talk about fuel additives. Are they necessary ? I have an extended warranty and I do not want to jepordize it. I noticed the owners manual talked about gas additives but not about diesel, any help would be appreciated.