The air filter is something so cheap I that I change it every 5,000 miles. Mine has been in use for nearly 6,000 miles now because I've been so busy to even get to it. It probably doesn't need any new one as the car's performance isn't worse by any means at all. But I'm sure its got oil all over it by now.
By the way, isn't 5,000 miles the recommended air filter change interval? And what tests were you referring to?