Tighter U.S. sanctions on Iranian oil planned for May are adding to a wealth of factors curbing the global supply of heavy-medium crude, driving up prices for scarcer barrels and setting up a stand-off between buyers and sellers. The new curbs on Iranian exports come on top of Washington’s earlier ban on Venezuelan crude and output snags in Angola, another big producer of the dense crude grades that best yield lucrative refined products like jet fuel. Refiners are also seeking more of the heavy sweet crude Iran and Venezuela once provided in abundance to produce low-sulfur fuel oil ahead of new shipping emissions rules due next year. U.S. officials say overall global oil supply will remain plentiful […]