Light, sweet crude for November delivery was down US$3.16 to US$90.72 a barrel in electronic trading on the New York Mercantile Exchange by mid-afternoon Monday in Singapore.
On Friday, the contract dipped 9 cents settle at US$93.88 a barrel.
In other Nymex trading, heating oil futures fell 6.92 cents to US$2.60 a gallon, while gasoline prices dropped 5.68 cents to US$2.17 a gallon. Natural gas for November delivery fell 13.1 cents to US$7.23 per 1,000 cubic feet.
In London, November Brent crude fell US$3.09 to US$87.15 a barrel on the ICE Futures exchange.