Detroit says it will ruin your engine. The EPA says it's safe.

Farmers say it's better than foreign oil. Oil companies say it's more expensive than gasoline.

But as Washington looks to compel refiners to blend more and more ethanol into gasoline, consumers are still left with the basic and critical question -- how much ethanol is safe to put in their cars?

Automakers warn the government's ethanol mandate could damage vehicles if it continues to grow.

"We just feel that it is not safe for the consumer. It's not safe for their engines," said Charles Drevna, executive president of American Fuel & Petrochemical Manufacturers.

The questions about ethanol arise after Congress first mandated it in 2007. Ninety-six percent of gasoline sold in the U.S. is now 10 percent ethanol, a high-octane fuel derived from corn. But under that bill and rules favored by the Environmental Protection Agency, refiners are now being forced to blend up to 15 percent ethanol into gasoline sold at stations around the U.S.

The auto industry, though, says E-15 -- as the blend is known -- corrodes pumps, fuel lines and injectors. And manufacturers say they won't cover damages caused by the higher blend.

The American Automobile Association agrees.

"Ninety-five percent of today's cars are not suited for E-15 based on what people who make those cars say," said AAA CEO Robert Darbelnet.

The ethanol lobby claims automakers and refiners are overreacting.

"E-15 has been sold in this country for the past nine months with no issues whatsoever. This is a lot of hysteria that's being driven by the oil companies," said Bob Dinneen, head of the Renewable Fuels Association.

While there have been no issues reported so far, the new blend has only been sold in a handful of stations in the Midwest. But refiners are mandated to use 13.8 billion gallons ethanol this year requiring the 15 percent blend. The EPA says it is safe for cars built after 2001, but acknowledges it is inappropriate for boats and small motors, including lawnmowers and chainsaws.

Automakers advise new owners not to fill up on E-15 and say doing so may violate warranty terms, leaving customers to pay costly repair bills. Toyota and Lexus even placed warning labels on gas caps and owner's manual instructions caution not to use E-15.

"We think ethanol is a pretty good product, up to a point," said Drevna. "But when Congress mandates such massive quantities that we can't put into the fuel system, that the autos and the lawnmowers people and the marine manufacturers are saying 'We won't warranty, we won't put this in our engines, there is a problem'."

Ethanol supporters dispute that and claim studies back them up. Ethanol blends of 25 percent have been used for years in Brazil with no ill effects on the same cars sold in the U.S.

"We support what the EPA did because we know that E-15 is safe for the vehicles for which they have approved," said Dinneen."Let the marketplace decide. Let consumers that have a newer vehicle, that want to use E-15, give them the choice. If they want to use E-15 because it is lower cost, because it's domestically produced, because it's the only thing we have that's going to reduce greenhouse gases, then they ought to have that choice."

After a lengthy comment period, the EPA is expected to decide soon whether or not to relax the blend mandate. Some lawmakers are considering similar legislation should the EPA fail to do so. Right now, it is a lobbying war pitting farmers against oil refiners and automakers.

"Look, this is pretty simple," said Dinneen. "It's about a battle for the barrel. Ethanol, renewable fuels, have been phenomenally successful over the last several years, and we are now 10 percent of the U.S. motor fuel market. And the refiners are saying, 'no more'. They don't want to see E-15 succeed. I think the American public still understands the value in reducing our dependence on imported oil and seeing more domestic renewable fuels used."