Oil rose to an all-time high of $80.20 on Thursday after Hurricane Humberto forced the closure of some U.S. Gulf refiners and stoked concerns of fuel shortages this winter.

Investors were also building up positions in the market -- their enthusiasm growing thanks to a market structure that can virtually guarantee favorable returns.

U.S. crude traded 21 cents lower at $79.70 a barrel by 10:36 a.m. EDT, having eased from its new record. London Brent crude also shed 21 cents to $77.47.

U.S. gasoline prices soared by 1.7 percent to $2.05 a gallon after Hurricane Humberto shut oil shipping channels and three refineries as it slammed onshore Texas.

There are so many things that affect the price of oil, Hasan Qabazard, director of OPEC's research division, told reporters on Thursday.

We have a storm working its way to American facilities. We have an economic crisis, so many things are affecting...prices.

Though quadruple the levels of 2002, the price of oil -- when adjusted for inflation -- is below the $90-a-barrel peaks of the Iranian Revolution in 1979 and the start of the Iran-Iraq War the following year.

Modest demand growth combined with no significant supply increases has caused oil inventories to decline sharply, creating backwardation in the oil forward curve, which is a very bullish signal, said Jeffrey Currie of Goldman Sachs.

In a backwardated market, oil for delivery in the near term is more expensive than that for later shipment. Investors make money by selling the more costly prompt oil and buying cheaper crude for later delivery.

The structure has been created in part by a belief among analysts and consumers that OPEC will not pump enough oil to satisfy demand for fuel this winter.

To try to wipe out the backwardation and soothe consumers, the Organization of the Petroleum Exporting Countries agreed a small supply increase on Tuesday.

But analysts said OPEC's pact to raise output by 500,000 barrels per day (bpd) from November 1 was not enough to reverse a rally that has lifted prices by 31 percent this year.

Market participants were also taking stock of crude inventories in top consumer the United States that fell 7.1 million barrels last week to the lowest level in eight months.

(Additional reporting by Neil Chatterjee in Singapore and Alex Lawler in Vienna)

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