Wow. I can't believe you've changed the oil every 3500 miles. (A) That's a lot of visits to the shop. (B) That's a tremendous waste of resources. Unless you drive in weird conditions -- like the Sahara -- I can't imagine you've done yourself or the environment any favors.My 2008 company cabrio has 107K. Oil changed every 3500 miles on Mobil 1 full synthetic and use a K&N air filter.