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Public service message from someone who went down this road before: it's worth studying up on the futures market. In particular, there's a nasty little thing called "contango" that effectively makes your investment worth less and less every month that you hold it and oil prices don't go up. Think of it as a rental fee for the tanks where they keep the oil held by investors. The upshot is that you can lose money on oi…
Is investing in a company like exxon a good proxy for oil price?
An integrated oil company like Exxon has activities that lose money when the price of oil goes down (drilling, exploitation) and others that make more (refining, retail). The price of oil dropped by about half during the past few months; Exxon barely budged.
A better proxy for oil price could be drillers (Diamond Offshore Drilling for example) but they're very far from perfect either. In general, it is very hard to track an asset which price is expected to be mean-reverting like the price of oil.