To replenish the rust inhibitors and other additives that might have gotten a little tired, how about draining a gallon or so and replacing it with a 50:50 mix of antifreeze and water? A 50% mix protects to -37 degrees F., so there's not much of a reason to use a stronger mix unless you're in Fairbanks Alaska or somewhere similar. A 70% concentration is the max, since pure antifreeze gets slushy at +19 degrees. It takes a little water in the mix to kick off the freeze protection.
Jerry
