Unleaded regular self-service gasoline dropped 2 cents in January to 96 cents a gallon, the second lowest-lowest January price since 1980, the California State Automobile Assn. reported. During 1987, gasoline prices in Northern and Central California and Nevada showed a 14-cent increase, the highest 12-month gain since 1980, according to the survey. Gas prices rose from 85 cents a gallon in January, 1987, to a peak of 99 cents in October, before dropping to 98 cents by the end of the year, the survey showed. The year-end price dip was caused by ample stocks of crude oil throughout the world, the automobile association reported.