The Brent/Dubai Exchange of Futures for Swaps (EFS) for August fell to $4.90 a barrel, down $1.20 from Thursday, valuations from two brokerages showed.

A fall in the EFS value would make crude priced on Brent more attractive to buyers than those pegged to Dubai. It would also allow more Brent-linked Atlantic Basin grades to flow into Asia.

Expectations of more supply in the market also caused the front of the Dubai curve to flip into contango, brokers said.

The spread for Dubai prices between August and September were valued at 2-4 cents a barrel in contango, from a 6-cents a barrel backwardation on Thursday's market close.

An oil major was bidding for the September-December spread at minus 30 cents a barrel, a broker said.

"So I reckon it's at least minus 5 cents down the curve," he said.

The International Energy Agency announced it would inject 60 million barrels of government-held stocks in the global market, immediately increasing world supply by some 2.5 percent for the next month and sending prices spiralling, with US crude prices erasing all of the year's gains.

Brent crude rebounded to $108 from a four-month low on Friday as traders gauged how much supply would reach the market from an IEA-coordinated release of emergency oil stockpiles.

"Sixty million barrels is really a drop in the water. There is no reason for the Brent structure and flat price to dump so much," a trader with a Western firm said.

Copyright AFP (Agence France-Presse), 2011