General Motors Corp. announced on May 23 that it will equip nearly all of its North American vehicles with a computerized monitor that will tell drivers when to change their oil. The GM Oil Life System, which is already available on Chevrolet Corvettes and some Cadillacs, has been found to double or triple the interval between oil changes. Vehicle owners are now usually advised to change their oil about every 3,000 miles, but the system could safely extend the mileage between oil changes to up to 15,000 miles.