"Recycled vegetable oil
Recycled vegetable oil, also termed used vegetable oil (UVO), waste vegetable oil (WVO), used cooking oil, or yellow grease (in commodities exchange), is recovered from businesses and industry that use the oil for cooking.
As of 2000, the United States was producing in excess of 11 billion liters (2.9 billion U.S. gallons) of recycled vegetable oil annually, mainly from industrial deep fryers in potato processing plants, snack food factories and fast food restaurants. If all those 11 billion liters could be recycled and used to replace the energy equivalent amount of petroleum (an ideal case), almost 1% of US oil consumption could be offset. Use of used vegetable oil as a direct fuel competes with some other uses of the commodity, which has effects on its price as a fuel and increases its cost as an input to the other uses as well."
Like I said before, I agree that something has to be done, and this may be viable at some point in the future, but at what cost? The waste oil comes from production of "food" items that are non-nutritious at best and deadly at worst. Petroleum is dirty, no doubt, and a limited resource to boot, but the technology to use other sources MORE economically is not yet in place. I work designing and building analytical equipment that researchers are using for biomass and coal gasification resarch as well as CO2 sequestration (greenhouse gas mitigation) and hydrogen storage for fuel cells, so I am kind of close to this.
I also have many years of experience in the analysis of petroleum, natural gas and pollution measurment (H2S, SO2, NOx).
Anyway, have fun in Czech, they have awesome beer there.