I am wondering what oil to use in my 2002 Ford Explorer Sport Trac which currently has 8200 miles on it. I drive an average of 300 miles a week, which 80% of those miles are city miles, and about 30 of those miles are spent towing a 2500 pound boat. I am currently running Castrol GTX 5W30 and a Motorcraft filter. From everything I've read on this board(great board by the way)I have learned that most 5W30 thins out to 20 weight after a few thousand miles. The owners manual says to run 5W30 and unfortunately says nothing about being able to run 10W30. I want to start extending my drain intervals, but have to use 5W30 because of the warranty. What oil should I be running in order to run 5K mile drain intervals? Thanks, Joe