DALLAS (AP) — Cheaper oil is leading to the lowest summer gasoline prices in years, and it is causing heartburn for oil companies and their shareholders. The reports from the two biggest U.S. oil companies followed weak second-quarter results from BP and Royal Dutch Shell. The companies have slashed spending on exploration and cut budgets to offset lower prices, but that has yet to create a sustained rebound in oil prices. [...] oil prices have faded again, with crude inventories remaining stubbornly high and the global economy mired in a funk. The average U.S.