Honestly - go to a reputed seller of fuel - test it - and if it runs fine - eventually - then stick with it - I really see no point in adding ANY 'additives' to a reasonably good fuel. there is simply no point. (snake oil) -
If you have just purchased the vehicle, with an unknown history, and it runs a bit like a bag of spanners, then just feed it decent known quality fuel and do the Italian tune up. It will be fine.
Old Wive's tales such as two stroke oil etc - are simply that.
The big issue with crappy fuels is diesel wash - your cylinders lose compression due to lack of the necessary build up of deposits leading to a good compression seal. There are SOME helpful products out there - but 99.9 % are snake oil.
Mind over matter - they sell them - it doesn't matter..........................
IF you have an issue - and IF decent fuel can help - then it will take many hundreds - if not thousands of Km to sort - the is no such thing as a quick fix.
There is absolutely and utterly no way on this tiny little blue and green planet that any person can differentiate ANY form of loss or gain of fuel consumption between individual - or even 'ten' 'fill ups' -
if they believe they can they are misguided - at best - and in reality utterly deluded - they simply do not understand the basics
There are some cases where the wear has become so pronounced that you get misfires at certain revs at on a specific cylinder. there is nothing you can do regarding this apart from changing the injector and praying.
.... preferably rebuild the engine instead of trying to blame the 'issue' on a 'known problem' with that engine - - cough cough - BS - as #'some# have been known to do here..... ----- leak off tests are also very subjective and mostly misquoted.
Also - ANY ronbox or synergy whatever can NEVER - EVER give you - improved - fuel figures. - simply NOT possible.
Enjoy
Joe