The phase-out of Nos. 4 and 6 heating oil officially began on May 23 with the requirement that all new boilers issued permits by the city must burn No. 2 oil. In accordance with Mayor Michael Bloomberg’s PlaNYC goal to make the air in New York City the cleanest of any big U.S. city, burning No. 6 heating oil will end by 2015, followed by the phase-out of No. 4 oil by 2030.