I'm on my 2nd tank of fuel since i bought the car 5 weeks ago. I had 3/4 of a bottle left of Millers from a previous car so added 2 shots in this 2nd tank, it doesn't seem to make any noticeable difference but maybe it will after a few tanks as this should clean the fuel system by then?
It claims to increase the octane of the fuel and also power - whether it's noticeable i'll see.
I read on other forums a few years ago that a lot of people recommend additives as some batches of fuel can be poor which causes poor running, etc so Millers, etc can help.