US benchmark West Texas Intermediate for delivery in December dived $1.08 to $81.01 a barrel on the New York Mercantile Exchange.
Brent North Sea crude for December shed 70 cents, settling at $86.13 a barrel in London trade.
Oil prices resumed their slide after Thursday’s sharp rebound on reports of OPEC kingpin Saudi Arabia’s supply cutbacks in September.
“Global petroleum markets have tipped back to the downside, reasserting the dominant bearish sentiment, with a reported decline in Saudi Arabian supply to the market shrugged off as the result of weak demand rather than a more constructive attempt to turn the market higher,” said Tim Evans of Citi Futures.
Schneider Electric analyst Matt Smith took a similar view: “Saudi supply skepticism combines with global economic concerns once more and crude falls.”
Barclays Research analysts said Saudi Arabia needed to do more to reduce its output after moving to accommodate rising Libyan crude-oil supplies.
“Saudi Arabian refining throughput and refined product exports reached record highs in August. Given a softening of global oil demand growth, the fragile return of Libya, and inaction from other OPEC producers, the kingdom’s oil supply is still ripe for an adjustment,” they said in a client note.
Investors also were closely watching also the impact of the spreading Ebola virus outbreak on global economies.
“Fresh concern over Ebola is really the last thing we like to see right now, which may have an overhang effect on oil demand,” said Desmond Chua, market analyst at CMC Markets.
A doctor who recently returned to New York after treating Ebola patients in Guinea tested positive on Thursday for the deadly virus, officials said, in the first confirmed case in the largest US city- AFP