For a disussion of contango, I would draw your attention to a thread
we had back on Feb 11, in particular my post #8.
Over the past three weeks, the oil futures contango has begun to flatten. Consequently, since my Feb 11 post above, USO has outperformed USL by 10%.
Closing prices Feb 11
Closing prices Mar 6
Returns Feb 11 - Mar 6
USO + 7.3%
USL - 2.8%
Ex market impact issues, the relative performance of these two ETFs is driven by the change in slope of the futures curve and not by a mechanical investment strategy.