Europe’s oil industry is once again generating cash even as crude languishes at half the price of three years ago. Yet companies remain vulnerable to a renewed downturn. The region’s top three — Royal Dutch Shell Plc, Total SA and BP Plc — can now cover spending from cash flow with oil at $50 a barrel. But BP predicts prices will drop this year, while Shell talks of a “lower forever” view. A fresh slump could put dividends at risk, and investors know it.