Robust demand for crude and a weak dollar have fuelled the rally from a dip below $50 at the start of 2007.
Adjusted for inflation, oil is now above the $101.70 peak hit in April 1980, according to the International Energy Agency, a year after the Iranian revolution.
The fall in the value of the dollar against other major currencies has helped drive buying across commodities as investors view dollar assets as relatively cheap.
It has also reduced the purchasing power of OPEC's revenues and increased the purchasing power of some non-dollar consumers.
OPEC oil ministers have noted that although prices are rising to record nominal levels, inflation and the dollar have softened the impact.
Some analysts say investors have been using oil as a hedge against the weaker dollar.
Since the Federal Reserve cut U.S. interest rates in mid-August last year and central banks pumped billions of dollars into financial markets to ease a credit crunch, oil and gold have risen.
Investment flows from pension and hedge funds into commodities including oil have boomed, as has speculative trading. At the same time, the credit crunch has brought some other markets, such as the U.S. asset-backed commercial paper market, to a virtual standstill.
Some of that money has found its way into energy and commodities, analysts say.
While previous price spikes have been triggered by supply disruptions, demand from top consumers the United States and China is a main driver of the current rally.
Global demand growth has slowed after a surge in 2004 but is still rising and higher prices have so far had a limited effect on economic growth.
Analysts say the world is coping with high nominal prices because, adjusted for exchange rates and inflation, they have been until recently lower than during previous price spikes and some economies have become less energy intensive.
OPEC SUPPLY RESTRAINT
The Organization of the Petroleum Exporting Countries, source of more than a third of the world's oil, started to reduce oil output in late 2006 to stem a fall in prices.
Fewer OPEC barrels entering the market helped propel the rally and consumer nations led by the International Energy Agency have urged OPEC to pump more oil.
At its meetings since December, OPEC has agreed to leave output unchanged, saying there is enough crude in the market. It next meets formally on September 9.
Few in the group believe there is much it can do to tame a market it says defies logic.
Supply of crude from Nigeria, the world's eighth-largest oil exporter, has been cut since February 2006 because of militant attacks on the country's oil industry.
Oil companies and trading sources have detailed 559,000 bpd of shut Nigerian production due to militant attacks and sabotage.
Oil consumers are concerned about supply disruption from Iran, the world's fourth-biggest exporter, which is locked in a dispute with the West over its nuclear programme.
Western governments suspect Iran is using its civilian nuclear programme as a cover to develop nuclear weapons. Iran denies this, saying it wants nuclear power to make electricity.
Iraq is struggling to get its oil industry back on its feet after decades of wars, sanctions and underinvestment.
Exports of Kirkuk crude from the country's north are stabilising as the system recovers from technical problems that had mostly idled the pipeline since the U.S.-led invasion of Iraq in March 2003.
Refiners in the United States, the world's top gas guzzler, struggled with unexpected outages which have drained inventories.