U.S. oil prices briefly dropped below $40 a barrel on Friday—hitting a six-year low that adds to pressure on pump prices for Labor Day road trips. But cheap gasoline isn’t a sure bet everywhere. Even as most drivers around the country are spending 25% less on fuel than they did a year ago, California drivers have missed out on the gasoline price windfall because of refinery outages. Many drivers in the state are saving less than 10%…