ETFs, designed for investors who cannot or will not buy and sell oil directly themselves, offer easy access and exposure to oil volatility because they are based on traded futures markets.
Many small investors such as pensioners, hobby traders, and savers on fixed incomes are attracted to oil ETFs, which can be designed to take advantage of price rises or falls.
But the volatility and structure of the underlying markets also make such investments dangerous for unwary investors.
"They can be quite dodgy," said a British woman who buys and sells oil derivatives from a rambling country house in western England using a trading platform run from a tax haven in the Caribbean.
"The smallest market difference can make you a lot of money - or a massive loss," she said, asking not to be identified.
One of the biggest risks for investors, particularly if they have "long-only" funds that buy and sell the front-end of futures, is the price structure of the underlying market.
If oil is rising in price and in short supply, futures markets may trade in "backwardation", with the front futures months at a premium to later months. When the front-month contract expires every four weeks, funds buy the second month and pocket the difference, earning a healthy profit.
But when futures markets have the opposite structure, a "contango" with the front months discounted to subsequent months, the process is reversed with funds having to pay out every month to buy the more expensive next months.
Both of the world's top crude oil benchmarks, North Sea Brent and U.S light crude, have been in contango for months, limiting any gains.
Oil funds are particularly attractive to many investors because they offer high gearing at a time when interest rates and fixed-income bond yields are at record lows.
One fund, VelocityShares 3X Long Crude ETN, has seen its net assets rocket to $698.4 million at the end of February from $1.6 million at the end of July last year.
The fund is leveraged three times and produces even bigger daily swings than the moves of up to 5 percent that the market has come to expect since prices hit six-year lows in January.
Carsten Fritsch, senior oil and commodities analyst at Commerzbank, says investors have flooded into oil derivatives over the last few months, helping to support prices.
"It was money flowing into oil ETPs (exchange-traded products) with stocks at record highs and bond yields at record lows," he told Reuters Global Oil Forum, an online chatroom for oil market professionals.
Oil funds are also seeing interest from computer-driven algorithmic and high-frequency trading.