I'm looking at getting the Volkswagen Golf TDI new for use with biodiesel but I was told by the dealer that using biodiesel would void my warranty.

My questions are:
1.) Would Volkswagen be likely to want to enforce this restriction if I were ever to take the car for repair under warranty?
2.) If they did want to enforce it, is there anyway they could prove I have been using biodiesel? Logically, wouldn't they just be able to tell by how clean the engine is (lack of buildup of glug etc..)?

Basically I want to know whether the warranty issue should preclude me from using biodiesel for the duration of the warranty period.

Thanks in advance