There used to be something called a Broquet fuel catalyst, although their website seems to have gone now.
I believe they had been about for a long long time. It was one of those weird things, the company claimed one thing with some sort of scientific explanation. But I was never sure if they really knew what it did or didn't do.
That said, I did spend some time researching the devices and my conclusion was, the only negative views were from people who didn't own one and had not used one. And there were literally hundreds of positive reviews and testimonies. Far to many and varied to have been bogus reports. This leads me to think that maybe it did genuinely offer some benefits, but likely for different reasons claimed by the maker and they had more than likely stumbled across the effect.
It's all too easy to call snake oil on something, especially as the vast majority of us are not industrial level chemists and are unlikely to be able to surmise what it may or may not be doing. Also with the advent of the internet, if something is rubbish, it normally gets called out as such and there is plenty of people who have laid out money happy to complain. I just couldn't find any evidence of this.
If memory serves, the biggest thing I saw people say was about better running, mostly on older vehicles with carbs and points.
Maybe it is snake oil, but lets face it, in this day and age we put ad-blue into diesels as an additive. As a "miracle" cure for emissions. If it hadn't come from the car makers, I'm sure most people would be skeptical.
Is your Land Rover still a petrol on carbs/points? Will be interesting to see if it still runs the same once you refit the tank.