http://www.importexporthelp.com/importing-cars.htm

A non-commercial, private use vehicle exported from the US may be returned duty free by verifying to U.S. Customs that it was previously registered and owned in the USA. A bill of sale for the vehicle or a state issued registration card may be used for verification. You must declare any accessories or repairs to your vehicle that were accomplished while out of country upon your return because they may be subject to duty.

Some countries only have leaded fuel available for vehicles. If leaded gasoline was used in your vehicle, you will need to replace the oxygen sensor and catalyst upon its return to the USA. You may get EPA authorization remove the oxygen sensor and catalyst prior to the vehicle shipping overseas and avoid replacing these parts upon your vehicle's return. The EPA authorizations number is (202) 564-2418. Upon return to the USA, the original oxygen sensor and catalyst will need to be reinstalled. However, if you provide assurance upon your return that the reinstallation will be performed, you will be able to reenter your US version vehicle without bond.