Contrary to what we might think, it seems that the oil life monitors in the GM cars are more accurate than we give them credit for. Here is a little info I found on them. there is plenty more on the link below.

The patented engine oil change technology involves computerized monitoring of engine revolutions, operating temperature, and other factors to optimize the change interval selection. The typical recommended interval for gasoline-fueled passenger cars and light-duty trucks is 3,000 miles (4,800 km) or three months, whichever first occurs, when outside temperatures are below freezing and trips are short. These conditions are considered severe duty. For ideal driving conditions, relating to long trips with mild outside temperatures, the interval can be expanded to 7,500 miles (12,000 km). Starting with the 2000 model year on certain vehicles, GM will raise the maximum mileage allowed for Oil-Life System-equipped vehicles to between 10,000 and 15,000 miles (16,000 and 25,000 km), depending on vehicle brand and engine.




http://www.noria.com/learning_center/category_article.asp?articleid=77&relatedbookgroup=Lubrication