SCAPPOOSE, Ore. -- Motorists in nine Oregon counties have started buying a blended fuel that is 10 percent ethanol. The rest of the state will join them over the next nine months.

The blended fuel became mandatory Tuesday in Multnomah, Clackamas, Washington, Clatsop, Columbia, Tillamook, Yamhill, Polk and Marion counties. Some of the counties have used E10 blends during the winter for years, under federal clean air directives.

By April 15, Linn, Lane, Benton, Lincoln, Douglas, Coos, Jackson, Josephine, and Curry counties will join the effort. By Sept. 16, all Oregon counties will be on board with the state's new renewable fuel standards.

The law's main objective is to reduce the use of fossil fuels and cut back on the greenhouse gases that contribute to global warming.

"A 10 percent blend isn't out of the ordinary," said Marie Dodds, a spokeswoman for AAA Oregon. "We don't see this as being a huge scary deal."

It's unclear what effect E10 will have on prices. Advocates of blending gasoline with ethanol hope it will drop the price by a few cents per gallon, but some retailers might charge a few cents more to cover preparation costs.

"I don't think anyone is predicting huge changes one way or the other," Dodds said.

Ethanol is a high-octane fuel and can improve performance. But E10 can reduce gas mileage by about 2 percent, according to ethanol trade groups.

Consumers will have to watch for the possibility of water in the fuel. Unlike petroleum, ethanol attracts the liquid. If gas stations haven't properly drained and prepared their tanks for the blended fuel, water could seep into a car's workings.

"It will run rough and stall," said Russ Wycoff, administrator of the Oregon Department of Agriculture's measurement standards division.

Distributors and retail stations have been told how to prepare for the changes, and Wycoff said he anticipates few problems.

Snowmobiles, motorboats, and lawnmowers and other yard equipment are fine with E10, but airplanes are not, Wycoff said.