This short film documents two young environmentalists using waste vegetable oil from various restaurants to run their fairly new VW sedan. It's good for them because emissions are significantly lower and the oil is free; the restaurant owner was glad to get rid of oil that is costly to dispose.
I don't know enough about mechanics to suggest doing exactly what they do, but it's something to look into -- a simple, affordable way to go green. Using vegetable oil to power a diesel engine has been documented elsewhere, and original diesel engines were made to run on peanut oil.
Happy Earth Day.