Later Models By participating in futures contracts that sat further from expiration, later ETPs like United States 12 Month Oil ( USL ) and PowerShares DB Oil (DBO) tempered the outsized negative roll yield that arose in front-month-only strategies. Products like USL, considered second-generation, simply ladder their assets across a number of futures contracts with varying expirations. As the Fed executed extremely loose monetary policy, hoping to prevent a deflationary spiral through the direct injection of capital to the economy, inflationary fears began to surface.