whether a coincidence or not... here is a bit of a feedback from my experience
- bought the vehicle (4.8is) which apparently was always ran on 95 octane fuel and no fuel additives used
- I used 97 octane every 3rd/4th tank
- in between that I use std 95 with redex additive to 'clean the fuel system'... not that I trust that much I was lead to believe that is the 'right thing' to do every now and then (as per recommendations on the bottle). E.g. my take was it won't cause any harm

- 95% of the time I only do short journeys in Lon of 2-5miles where engine temp does not even reach the optimum level... I know it is not great for any motor whether petrol or diesel but I do have occasional blast on M4/M40 for piece of mind and to get a grin on my face when I open up the throttle :xmasgrin:.
so in 4 months of ownership I noticed...
- fuel additives certainly help a car to work better... smoother running.
- equally I noticed that 97+ octane is defo better... however I could not justify the cost as my vehicle is juicy in it's own right and that couple with abundance of power/torque so not really a need for it.
- and the last but not least... with combination of 97oct and/or additives I also noticed that I get the occasional blue smoke at full throttle?! a coincidence or not but it may well be that something had happened since I first started using them two... 'kx' suggested it maybe a Valvetronic, whatever it is the issue could have been triggered by those or at least i'm lead to believe that. While I appreciate that other factors would/could influence it... I still enjoy my vehicle and I think it is the great piece of eng... complex, large, heavy, thirsty with 'decent' maint. bills but worth every penny

(I guess only Jag XJ would qualify in this category too).
a bit of a rumble but hope that helps...