U.S. crude fell 30 cents to $100.34 a barrel by 1142 GMT, after rising by more than $2 to as high as $102.84.
London Brent crude was 53 cents down at $97.64.
Analysts in a Reuters poll expect the government data to show an increase in crude oil stocks and drops in oil product inventories when it is released at 1435 GMT.
The analysts forecast a 2.4-million-barrel increase in crude oil inventories in the week to Sept. 26. Distillate stocks were seen falling 1.2 million barrels with gasoline inventories seen down by 1.6 million barrels.
Losses were limited, however, by broader rallies on global markets due to hopes that a revamped U.S. financial rescue package would be passed.
The U.S. Senate will vote on Wednesday on a new version of the $700-billion bailout package for Wall Street, rekindling hopes that the credit crisis can be eased before claiming yet more banks and causing further damage to the global economy.
"There is a strong belief that the U.S. Congress will pass the rescue plan in a few days, so that's supporting prices," said Ryuichi Sato, an analyst at Mizuho Corporate Bank in Tokyo.
On Tuesday, U.S. crude settled $4.27 higher at $100.64, rebounding from Monday's sharp $10 loss that was in reaction to the rejection of the original rescue plan by the U.S. House of Representatives.
Asian and European shares rose, led by banks, on Wednesday. U.S. stocks soared on Tuesday, with the Standard & Poor's 500 index marking its biggest one-day gain in six years.