A difficult subject, But why? Of course, I could recommend that you refrain from using any fuel- or oil additives at all. That would attract the readers who have used them to treat engine problems successfully, e.g., odd noises coming from the valves under certain circumstances, or those made by Diesel engines.
Either that, or somebody working for a company, who has been selling such additives for decades, would write to report on tests that neutral experts have carried out, where, since using XYZ-additives, less pollutants have been verified.
If I were now to say, that I am doubtful about such products, I would immediately be criticized as being hopelessly out of date and ignorant. Thereby, all I'm thinking about, is the sinfully expensive advertising which can be seen all over the Formula-1 racing cars, which automatically answers the question of why these product are nearly always pretty expensive.
No, you don't have to worry, I won't leave you bewildered, however, first things first: Are you adding something to the fuel or to the oil? Both together certainly doesn't make sense. And if you're using an additive to the fuel and also using premium petrol, that can't be recommended either.
Now, one more question? Do you actually have a problem with your car, or do you simply want to prevent one occurring? In the case of the latter, I wouldn't use an additive. So, the last question for the time being: Are you planning a longer trip with your car in countries with an uncertain fuel-quality? If the answer is yes, then I would get as much advice as possible, e.g., from people on the internet who have experience in this type of undertaking or are themselves running a car similar to your own in the region.
I imagine by now, that the basic principle should be clear: Under no circumstances take any action if there is no reason to do so, that also means not using the more expensive fuel. OK, you may be assuming a possible fuel-saving, however, consumption tests always leave one with the suspicion, that using the gas pedal more gently, could quite well have the same effect.
Exhaust gas test-results, which are lower than the pretty strict legal directives, wouldn't impress me all that much. Now, let's assume a problem with the car does exist. Try one of the tips you've read about, which may be of help, really, only one, and test this one so long, until you know for sure whether it has helped or not.
Give it some thought, as a rule, your car has been expertly put together and has been tested using products approved by the manufacturer. If in fact, there were the ultimate petrol- or Diesel additive, at least one brand of petrol would already have it and would advertise it massively. By the way, the ingredients used to improve certain qualities, are well known to the experts in the field. Indeed, one is almost tempted to warn, that by adding additives to an expensive fuel, may even make matters worse.
I must admit, that in the past I have successfully used a radiator sealing compound. I would probably have used it again as well, if I hadn't, quite by chance, come across a thoroughly gummed up heater-radiator. A heater-radiator, any idea where it's installed? Deep down in the dashboard, as far as the workshop hourly rates are concerned, the most attractive place. 02/15