Hedge funds and big Wall Street banks are taking advantage of loopholes in federal trading limits to buy massive amounts of oil contracts, according to a growing number of lawmakers and prominent investors, who blame the practice for helping to push oil prices to record highs. The federal agency that oversees oil trading, the Commodity Futures Trading Commission, has exempted these firms from rules that limit speculative buying, a prerogative traditionally reserved for airlines and trucking companies that need to lock in future fuel costs. The CFTC has also waived regulations over the past decade on U.S. investors who trade...