I am not going to debate what might or might not be good for plonking into the tank of a modern cdi powered vehicle. dealerships generally put an additive into the fuel tank whenever they service a vehicle, but the warranty is quite clear and it goes along with Iain's recommendation. Our warranty clearly states no additives and only fuel that complies with certain stipulations.
Like a few others here I have a Service Plus contract so have no opinion about good or bad, but I will not risk invalidating a warranty.
John