Hard answer... In 2004, the passat had many engines... I'll break it down.
If you had the 2l TDI diesel engine, use Synthetic Mobil 1 5w40 diesel oil.
If you have the 1.8T turbocharged 4cyl engine, use Synthetic 0w40 or 5w40
If you have the 2.8l VR6 6cyl engine, use 0w40.
If you had the 4l (VV)W8 engine, use 0w40.
So pretty much, between 0-5w40 will be ok regardless of the engine.