In my Monte i've always used the OLM which goes off every 7-9K miles. I also use synthetic oil.

In cars that don't have the OLM, I change the oil every 7,500 on synthetics, and if I used conventional, i'd change it every 5,000 miles.

 

I bet if you showed up at that same dealer with cash in hand and said it's been 3,000 miles since you changed your oil, they'd be very willing to change it for you. Since they're doing it for free, however, they only want to see you every 6,000 miles. I change my oil every 5,000. You really need to keep an eye on the oil level if you're going 5,000-6,000 miles between changes.