New stickers warning consumers to only use E15 in flex-fuel vehicles went up on gas pumps last week in Iowa and most of the country as the calendar flipped from May to June.

In other cases, gas stations ceased offering E15 due to an Environmental Protection Agency (EPA) regulation that limits the use of 15 percent ethanol blends in nearly 70 percent of the country from June 1 to Sept. 15.

"This regulation handcuffs fuel retailers who want to offer E15 to customers year-round," said...