The best treatment is to change it and filter every 3000 miles. Use a premium 10W30 winter in North Carolina, 20W50 summer.

Buying synthetic and then running it 10,000 miles to 'save money' is not a good idea. The Flexible Service Systems (FSS) are there because the car manufacturer gets 'dinged' by the EPA and tree huggers for the 'waste generated' by the car from oil changes hence the push for very long intervals--and remember they want you to have to buy a new car sooner-and your used one to be junked and not go to me in great condition. So long as it makes the warranty period, and the lease ending resale is not reduced, they are happy