I would tend to doubt. Did you know in most states we blend butane in the gas in the winter? We can't do it in the summer due to RVP limitations in most urban areas.

Summer formula... I love hearing that. The "summer" gas is just a lower RVP. This is to control the evaporation of the gas during the warmer months. Ever find a plastic swollen gas can due to the heat? I keep my mower/genny gas can cap open in the summer just for this reason.

Read up more on RVP. I run these tests daily.
http://en.wikipedia.org/wiki/Reid_Vapor_Pressure

 

In the winter (Virginia) we run a more volital winter mix gasoline. Makes a little less Hp so my gas mileage (as well as others I talked to) dropped a few mpg. As soon as the spring came the MPG jumped right back up.

 

More volital?