Oil turned lower Thursday on profit-taking after bouncing off four-month lows.
U.S. stockpile figures had helped oil surge after easing some glut concerns.
U.S. crude was down 33 cents,or 0.88 %, at $40.47 at 06:45 ET after a high of $41.41. Brent crude was down 1.28% at $42.55 after a high of $43.65.
U.S. crude jumped above $40 on Wednesday after the release of the latest weekly Energy Information Administration stockpile data.
U.S. crude stocks rose by 1.4 million barrels when a draw of a similar magnitude was expected.
At 522.5 million barrels, U.S. crude stocks remain historically high.
However, gasoline stocks fell 3.3 million barrels when a modest rundown of 200,000 was expected.
The dollar index was slightly higher. A stronger dollar normally depresses demand for oil.