In the following video, Motley Fool energy analysts Joel South and Taylor Muckerman discuss two major U.S. refiners, Marathon Petroleum (MPC 0.80%) and Valero Energy (VLO 0.27%), after earnings were released, and investigate why the shrinking WTI-Brent spread may be depressing their stock prices, despite strong quarters for each. Joel gives investors the key things to watch to know if margins in these companies, and across the sector as a whole, may slip downward as the crude price spread continues to narrow. He also ponders how you'll know whether you should stay in, or if it's time to get out.