EXXON <XON> EXTENDS MID-GRADE UNLEADED MARKETING Exxon Co U.S.A. said it is extending marketing of its mid-grade unleaded gasoline, Exxon Plus, into the Houston and Dallas/Fort Worth metropolitan areas beginning early next month. Exxon Corp's domestic subsidiary is already marketing the 89-octane unleaded gasoline along much of the East Coast. As it introduces the third unleaded grade, Exxon noted, it no longer offers a leaded gasoline in the market, except in some rural areas where leaded gaosline will be made available to distributors who have a substantial need for the product to serve customers whose equipment was designed for leaded fuel.