Similar corporate governance reforms haven't helped the returns of other oil giants. Royal Dutch Shell and BP have both split the roles of chairman and chief executive, without any discernible benefit to shareholders. Since 2006, when Mr. Tillerson assumed the top roles at Exxon, the company's stock has climbed 57%, compared with 12% for Royal Dutch Shell and 4% for BP. Over the past 10 years, Exxon has consistently outpaced those rivals and the industry average in annual average returns on investment.