Hi. I own a 2007 Matrix and it has about 8,500 miles on it. How long should I go between air filter changes? The dealer tried to get me to change it when I last had the oil changed (at the 6,000 mile mark), but I didn't want to spend the $60. I recently lost my job, and don't want to change the filter unless I have to. Should I get the air filter changed soon? I live in an urban area, so I'm not driving along dusty roads. When should I get the filter changed? Thanks!