Wal-Mart Stores Inc.’s auto centers, the car-maintenance areas attached to many of its stores, are getting a service overhaul.To improve the experience and entice more customers, the company is dedicating staffing and training programs to the department. A new class of salaried assistant managers will be assigned to the auto-care centers, where they’ll oversee merchandise and services, said Kory Lundberg, a spokesman for the Bentonville, Arkansas-based company.The changes are meant to yield benefits beyond the auto centers.