MH370 vanished an hour into a night flight March 8 from Kuala Lumpur to Beijing with 239 people on board, mostly Chinese nationals.
No trace of the Boeing 777 has been found despite probably the biggest and most expensive search in aviation history.
The oil slick samples were collected by the Australian vessel Ocean Shield near the search site 2,170 kilometres north-west of Perth.
An underwater probe launched from the Ocean Shield has trawled 90 square kilometres of the ocean floor where computer modeling suggests the plane entered the water.
The Bluefin-21 submersible was sent down for the first time Monday after searchers gave up hope of hearing more signals that could come from the emergency beacons attached to the fuselage.
The search area was further refined by analysing the four acoustic signals picked up by the Ocean Shield.