Less than a year ago major shale firms were saying they needed oil above $60 a barrel to produce more; now some say they will settle for far less in deciding whether to crank up output after the worst oil price crash in a generation. Their latest comments highlight the industry's remarkable resilience, but also serve as a warning to rivals and traders: a retreat in U.S. oil production that would help ease global oversupply and let prices recover may prove shorter than some may have expected.