The biggest question in oil markets is what OPEC should do once its deal to cut output ends next spring. Goldman Sachs Group Inc. has an answer, but it requires a bit of balancing. Oil prices have slipped 8 percent since OPEC and its allies agreed on May 25 to keep production reduced until next April amid uncertainty about what they will do after that. For Goldman Sachs, the organization’s challenge is to reduce supplies enough in the short term without boosting prices so much that rival producers get back to work. To juggle those objectives, OPEC should make deeper production cuts now and simultaneously warn it will revive output sharply once the surplus clears, according to Jeff Currie, the bank’s head of commodities research in New York. […]