The unmanned sub's first two missions were cut short by technical problems and deep water, but the Bluefin 21 finally managed to complete a full 16-hour scan of the silt-covered seabed far off Australia's west coast, the search co-ordination centre said.
While data collected during the mission, which ended overnight, were still being analysed, nothing of note had yet been discovered. The sub has now covered 90 square kilometres (35 square miles) of seafloor.
Officials said the oil analysis done in the western city of Perth came up empty when the samples tested negative for aircraft oil or hydraulic fluid. The oil was collected from a slick about 5.5 kms (3.4 miles) from the area where equipment picked up underwater sounds consistent with an aircraft black box.
It was hoped the oil would be evidence that officials are looking in the right place for Flight 370, which vanished on March 8 while en route from Kuala Lumpur, Malaysia, to Beijing. Searchers have yet to find any physical proof the sounds that led them to the ocean floor where the Bluefin has been deployed were from the jet.
Twelve planes and 11 ships were scouring a 40,300 sq km patch of sea.