Notwithstanding the inconveniences, my wife, Ginger, is as enamored of this experiment as I am. She has claimed the Jetta as her own, but has volunteered to help with the mechanical work to convert another car for me. She calls the experience the “Noah project,” named after our 11-month-old grandson, who she hopes will benefit from a better world if others do the same thing.

Despite the obvious benefits of using a fuel that contributes to the nation’s energy independence, that is relatively cheap and that can be burned after having already served its original purpose — cooking food — it is worth noting that vegetable oil is unlikely to replace petroleum anytime soon.

As the number of conversions rises, users will eventually soak up the supply of used cooking oil. As with ethanol and other agriculture-based fuels, it remains to be seen whether growing soybeans is an efficient way to produce nonpetroleum fuels, since the farming process consumes large amounts of fuel and chemical-rich fertilizers. Also, growing crops for use as fuels could have unforeseen effects on the prices and supplies of food.

The territory is uncharted in other ways, as well, as seen in the comments posted in an online forum sponsored by Greasecar.com. Take the case of “Chase,” a Massachusetts resident who was perhaps not as careful as he might have been in the storage of his cache of oil. “No BooBoo to be seen,” Chase wrote, “but this AM there was a really big black bear with its snout in an opened and overturned 5 gal. Luckily the grease spilled onto my gravel driveway, so it should soak in soon enough. Also lucky she did not get into the 12 other buckets! Some arm waving and yelling sent it packing.”

A frequent question about vegetable oil is whether cars run more sluggishly on it. Consider “TDIGuy,” who sought advice on whether a blown head gasket could have resulted from “running a VW too fast for too long” and “hitting the gas pretty hard.” After receiving some helpful comments, TDIGuy came clean: “When I said speeding a little, and a little hard on the gas, I was actually trying to hit 140 in the car. Managed to get 130, but I think I put too much stress on the engine.”

Image A filtering system keeps impurities found in used cooking oil out of the Jetta's engine. Credit... Jim Norman/The New York Times

A common concern about converting a car for vegetable oil is that it could harm the engine. But some people who have done conversions say they’ve seen no damage, even after many miles. Phil Gibbs, a New York City firefighter who makes a 75-mile commute twice a week from his Putnam County home, said he had driven his 2002 Jetta 75,000 miles on vegetable oil with no trouble.