Saudi Arabia has challenged other big oil producers to join it in output cuts, saying it will back actions to shore up prices next year if these are mirrored by rivals both inside and outside the cartel. While the proposal from Opec’s de facto leader sets a high bar for a deal and is unlikely to lead to a cut at Friday’s meeting in Vienna, it paves the way for a possible agreement in the future. It also marks a softening in tone from the world’s largest oil exporter, which led the cartel’s shift in policy a year ago, arguing that keeping the taps open would put pressure on high-cost rivals. The move upended the oil industry and triggered the biggest price slump in at least a decade. But in recent weeks Riyadh has sought to counter the mantra of “lower for longer” prices that has taken hold in the industry and led to hundreds of billions of dollars of investment cuts by oil majors.