I usually class all of these additives as "Snake Oil" and don't bother with them but I remember MDP posting about this stuff when he gave up his S/C and bought the Audi. I bought a bottle and then promptly sold the V6 and haven't put any into the V8 because I think the engine is still a bit new to need these things.
So what is the verdict - useful additive or Snake Oil?
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A Carbon Footprint is the mark a lump of coal makes when it's dropped on the ground.

TDv8 HSE - Stornoway Grey with ebony and straight grain walnut wood-a-like interior with towpack, Tasmods. The best car I have ever owned
Now departed - Giverny Green TDv6SE with Aspen and Parchment interior - nearly the best car I have ever owned
Staying - Epsom Green Defender 90 for playing off road