I have the same situation with our '03 Cavalier. Most of its miles are from my wife driving 15 miles each way to work, almost all highway. I could probably get away with the 7.5K interval but the oil nut in me won't stand for it. I usually dump it between 3 and 4K. In fact, I just finished giving it a new filter and 5 fresh quarts of Havoline 10w30.