Once upon a time, the straight six was a dominant engine design. Jaguar put them in its best cars, Jeep built its late 20th century reputation upon them, and almost every humdrum family car or pickup truck in America had one as standard—then they nearly died out. For years, V6 engines were killing off the straight six, also known as an I6 or inline six, and the entire design appeared destined for oblivion. But Mercedes-Benz has performed a resurrection. It brought back the straight six in the form of the M256, an entirely new engine design, to replace many of...