I occasionally chuck about 400ml of 2 stroke oil in my old PD130 A6 when I tank it up.
After about 100 miles it becomes noticeably quieter and picks up about 3-4mpg over the tank.
Bum-Dyno says it feels a bit more responsive too but I expect thats just the placebo effect.
I only put it in when I have some leftover from work. Usually buy a 500ml bottle of the cheapest green mineral stuff use about 100ml to mix up some 2 stroke for some kit then dump the rest in the car.
I keep meaning to buy a couple of litres and dose it over 5 tanks and see if there are any long term benefits but I've not been doing the miles nor do I have the motivation.
I do however recall the first MOT I had with it the emissions whilst a pass were close to the limits.
The 2nd MOT happened to be when it had some 2-stroke in as I was concerned it would fail, to my surprise it was about a third of the previous years test!
Same garage, same tester, same gear and had been serviced both times prior to MOT by myself.
With the above in mind I gave it the 2-stroke treatment prior to it's most recent MOT and the emissions were similar to the previous (different garage this time).
Read into it what you will, I'd not hesitate to put it in my A6 all the time if I could be bothered as it does seem to make a difference (at least on the trusty old PD130), however YMMV and I take no responsibility should your car self destruct the DPF clogs or you put too much red coloured 2-stroke in and get a tug from the Gestapo.