OAKLAND -- The first day of a free alternative fuel giveaway started Tuesday with a trickle of motorists lured in by curiosity and the promise of a bargain: Five free gallons of biodiesel or an ethanol-based blend called Flex Fuel.

"It's an incentive for drivers to stop," said Emily Shellabarger, a spokeswoman for Propel, the Sacramento-based fuel company that opened a self-serve fueling dock at a Chevron station at 350 Grand Avenue.

The opening came amid the BP oil-drilling disaster and on the second Spare the Air day in the Bay Area.

"People are excited to have another choice," Shellabarger said.

Propel's Flex Fuel is 85 percent ethanol and 15 percent petroleum. The biodiesel is almost all petroleum-based, with 5 percent coming from biofuels.

"Our pumps are a little bit different looking and it's a bit confusing, but we're an independent company," Shellabarger told a gray-haired woman who pulled up in a Chrysler Sebring.

The Sebring was not equipped for Flex Fuel although there are about a million cars on the road that are, including the 2009 Hummer H2.

Propel provides a list of Flex Fuel-ready vehicles at the pump. Motorists can also tell by the manufacturer's yellow gas cap. They can check the website www.propelfuels.com/driveE85.

The regular per-gallon price for the Flex Fuel is $2.49; the biodiesel is $3.19 and comes from a supplier called Western States Petroleum in San Jose. Mileage is comparable, Shellabarger said. Customers pay at the Propel pump with a credit or debit card, or an employee fleet card.

The pace of cars at the pump quickened by noon: A shiny Chevrolet Suburban, a 1982 GMC Sierra Classic pickup, a new model Volkswagen Golf. But a woman returning a Zipcar was unable to take advantage of the deal because the Toyota Matrix she rented was not equipped for alternative fuels. No Hondas are Flex Fuel equipped.

The pump is open 24 hours a day, seven days a week. The promotion ends Thursday.

The Oakland location is Propel's third in the East Bay. Other sites include Fremont and San Jose, where a second location is slated to open soon. A Berkeley location also is in the works.

Shellabarger said Propel would like to use non-corn-based ethanol, as well as less petroleum in the diesel blend. But 5 percent is the maximum amount in a blend that California allows to be stored underground at filling stations, she said. The other roadblock, she added, comes from manufacturers that recommend no more than a 5 percent blend and will not honor warranties otherwise.

"We're working with what we have," she said "It's not a silver bullet. But it's a step in the right direction."