Winter blend gasoline has more butane. It evaporates in heat a little more quickly than summer blends and thus, all else equal, pollutes more, hence the reason it is usually only used during the winter time.
Thanks Lilo. I really had never heard of it before.
TIS
"Mankind must put an end to war before war puts an end to mankind. War will exist until that distant day when the conscientious objector enjoys the same reputation and prestige that the warrior does today." JFK
