(Bloomberg) — American pipeline operators have begun asking oil producers to voluntarily ratchet back their output in the clearest sign yet that a growing glut of crude is overwhelming storage capacity.Plains All American Pipeline LP, one of the biggest shippers of crude in the U.S., sent a letter this week asking its suppliers to scale back production. The notice came from the company’s marketing unit that buys and sells oil to customers. A Texas oil regulator said Saturday that drillers were getting similar notices from pipeline operators.The messages signal the oil market is fast approaching the moment traders have been warning about — when crude supplies overflow storage tanks and pipelines as the coronavirus pandemic drags down oil demand by the most in history.“We are