I have a 2007 Silverado Classic with 35K. Normally, this vehicle sees less than 100 miles a week, and the oil level does not change visibly between oil changes. Over the past 6 weeks, I have been communiting 100 miles a day, and driving 600 miles most weekends. When I checked the oil, it was a quart low. Since this is the first time I've seen a visually detectable difference in oil level between changes, I wanted to ask whether this is normal, or a cause for concern. Thanks.