Brent crude also dipped below $80 a barrel but ended at $80.05, $1.36 or 1.7 percent lower.
USA inventories rose 6.49 million barrels last week, according to government data released Wednesday, more than twice the amount forecast in a Bloomberg survey.
US lawmakers pointed the finger at the Saudi leadership over the disappearance of the Saudi critic, suggesting sanctions could be possible.
The exchange operator has been adding new USA oil contracts because it has seen a lot of interest in the spreads to WTI, Johnson said, noting that there is a lot of liquidity for the new derivatives trade for new US grades.
But now, with the U.S.'s withdrawal from Iran's nuclear deal in May and re-imposition of sanctions against Iranian economy, the issue has come again under the spotlight as an approach to counter the new round of sanctions.
Curry, Durant lead Warriors past Thunder in festive opener
Meanwhile, big man Steven Adams could also miss the opener due to lower back stiffness, which kept him out of Monday's practice. Al Horford blocked five shots for the Celtics , who shot 43.3 per cent from the field and were 11 of 37 from three-point range.
In global markets, stocks in Asia fell on Friday as global sentiment soured on issues ranging from trade worries, Italy's 2019 budget, higher USA interest rates and growth concerns in China that led to a slump in Chinese shares in the previous session.
For the week, Brent crude was 1 per cent lower while WTI futures were down 3.4 per cent, putting both on track for a second consecutive weekly decline.
On the other hand, Trump could opt to take a middle ground in an effort to prevent oil and gasoline prices spiking ahead of the midterm elections. Saudi Aramco will buy a stake in an upcoming Chinese mega refinery, its latest investment in the world's biggest oil market as it pursues a strategy of securing avenues for oil sales.
Continued weakness in global equities was also a weight on crude this week, analysts said, underlining worries about global growth prospects and dampening demand for assets perceived as risky, including most commodities.
As the US sanctions on Iran's oil sales targets the Iranian state-run sector, Iranian government chose to offer oil in the stock market to permit the private sector to export the product to foil USA sanctions.
The US has asked many countries including India to bring the oil imports to "zero" level from the Gulf nation by November 4 this year or face the outcome of sanctions. The government chose to sell 500,000 barrels of heavy crude at a fixed price.
But while Saudi Arabia and Russian Federation have boosted output, some OPEC officials are anxious about a global oil surplus.