In my normal gas (Shell V-power 91) I didn't notice any mileage improvement from FP60. I'm not sure if it's making more of a difference in other brands. Right now Esso 91/92 gets MUCH better fuel ecomony than V-power. I could be looking at a V-power+FP60 does nothing, Esso is better and Esso+FP60 is much better type of scenario. I've currently written about this in another thread in this forum. The difference in fuel economy is NOT subtle but very pronounced.
I continue to use FP60 for the cleaning aspects, if nothing else. I didn't buy it expecting for it to pay for itself but if it results in better engine condition and cleaner operation that's fine with me. I ordered some FP3000 from that limited run. It is darker red, thinner and treats twice as much gasoline for the same amount of FP. It smells similar but yet different. I haven't run it yet.