Let me preface this by saying that oil / fuel additives are
very subjective and exploration of them is at best quasi-scienticfic, but much more likely superstitious pier-reviews of snake-oils.
However, a long time ago I got very anoraky and did some data logging & calculation crunching, and found that running Shell V-power diesel basically trumped any combination of fuel + additive in terms of mpg delivered over a tank + perceived "drivability". With fuel additive in V-Power there was no discernable benefit, with baseline premium brand fuel + the EHN the cost per mile wasn't as good as V-Power diesel, with supermarket diesel + EHN the driveability was
almost as good as the V-Power, but the cost per mile (based on MPG) wasn't quite as efficient as the V-Power diesel. I ran a couple of tanks of BP-Ultimate diesel in another vehicle, but I have a natural brand loyalty / bias toward shell, so most of my testing was done with shell. For baseline fuel I used mostly Morissons & some Asda fuel, and for "baseline branded" fuel, I used esso regular and shell "fuel saver", If you cannot get V-power diesel, or maybe an alternative like BP Ulitmate would also be up to mustard, I'd recommend a decent branded fuel and a healthy slug of 2-EHN in the tank. this
EHN / 2-EHN I keep referring to is 2-Ethyl-Hexyl Nitrate - a "cetane booster" available on eBay, and generally forms the basis of products like millers additives. Wikipedia cetane booster:
https://en.wikipedia.org/wiki/Cetane_Improver
For oil additives, I run a cocktail of decent synthetic oil + with a slug of Lucas Oil Stabiliser in there and a dose of ZX1 additive. But again, this is subjective and the selection of products I've recommended will be at best based on my own perceptions / experiences, at worst me advocating brands I have unwavering loyalty to / prejudice towards.
Of everything I've said above, 2-EHN is scientifically proven, the rest is subjective.
