The only myth that has any truth was that lower octane fuel created engine "knock" and that is 100% true... in older carburated motors. They cannot as accurately regulate the air/fuel mix going into the engine as a computerized fuel injector. Carburetors need adjustment, as a part of regular maintenance, to keep the air/fuel mix as accurate as possible. So many times, these adjustments were not made regularly causing too much fuel to be mixed with the air. When this happened the gasoline would not burn completely soaking into carbon deposits. This would cause a premature ignition of the gasoline due to the intense heat in the engine cylinder creating "engine knock." When this happened, people would change to the higher octane/slower burning gasoline to resist the premature burn, thus minimizing the knocking problem. And it worked. Good solution. But now fuel injection and plain smarter engines has eliminated that need. And if you happen to see a small increase in MPG (I don't) it is negated by the higher price. Best bet is to do what they are all saying and use what is recommended.