Anti-freeze in my four year old GMC is, of course, four years old as well. The manual says it should be changed. Isn't there an over-the-counter chemical that will rejuv the existing anti-freeze? I've heard that ani-freeze never wears out but the anti-rust and anti-corrosion additives do. Wonder what the readers would recommend? Change the anti-freeze or rejuvenate the existing anti-freeze and what brand would be a recommendation. Thanks DG