Yikes. You may change the oil regularly, but I'm guessing someone else didn't before you came along. You might try a product called AutoRX that's supposed to help gently clean up dirty engines, but that one may be beyond anything in a bottle will help. I say, if it is running fine and not burning oil, keep doing what your doing.
Either way, check that PCV system. Replace the valve and the breather filter in the air cleaner housing. Make sure all of the lines are free from obstruction.