About that additive: as you just pointed out, it's impossible to control enough variables to be able to actually tell whether or not it does anything! (The manufacturer knows this and counts on customer ignorance for repeated sales.) Add to this the fact that your car is going to be getting better mileage anyway as the ambient temperature goes up.
The manufacturer laughs all the way to the bank with your hard earned money. Yet they haven't submitted the product for EPA-sanctioned testing.
Save your ten bucks! Use it for gas instead, and I
guarantee it will take you nearly 50 miles.
(At $3.75 / gallon prices.)
The maker of that product knows if they claimed the same thing on their packaging, they would end up in court, and lose.