My wife has a 2003 Legacy L (2.5) she has had since it was new. It currently has 86000 miles. She got a new job this year and has been driving about 620 miles / week since September. I have noticed that the car is low about a quart of oil after about a month following an oil change. It has a very minor leak, so I know that that much oil isn't dripping out. How do I know if it's burning oil or is this normal when putting that many miles on the car? Also, would it be a good idea to use a full synthetic oil or even a blend?