With oil costing 40-50 $ per barrel, it's so easy to forget that as recently as last summer, the price was pushing 150 $.
The price didn't drop because supply went up. It wasn't a speculative bubble either.
It dropped for the simple reason that worldwide demand is down sharply.
Which means: If/when the economy recovers, the oil price will quickly explode, and it won't stop at 150 $:
We know that the "cheap 'n easy" oilfields are in decline.
While there may be lots of "hard 'n difficult" oil still in the ground, it will be much more expensive to get it out.
And in addition, even if we throw lots of resources at it, it will take time, and the amounts we can get out in the short run are limited.
So we'd do well to remember that any meaningful economic recovery will immediately cause resource-constraints.
This means higher oil prices, and (among other things) lower demand for European and American cars.
(While oil supply will stay more or less at the 2007/08 level during the next 5-10 years, and will start to decline thereafter, it is a sure thing that emerging market demand will go up, so the market can only be balanced if mature economies are sufficiently "crowded out" by a much higher price)