Fuel Doctor is heaps cheaper compared to Wurth CRD Additive.
I didn't try the Fuel Doctor yet. From what I see in the video they have on their website; it looks and most likely it is a good product. Backed by Castrol, it should be safe to use in CRD with the DPF, Greg is using it in his Golf GT and has no problems with the DPF, it could be worthwhile to try.
Bookmarks