While it's not an additive, an excellent choice for the GL1800 is Evans NPG+ waterless coolant. The system must be completely drained and flushed and when Evans is added, you have a much higher boiling point and because it's waterless it won't produce steam and reduces operating pressure. This product has been discussed before on this post and most who have used it a very pleased. It's supposed to last for the life of the vehicle. I've had it in my bike and cars for over 6 years without a hiccup. I originally looked into it because of the famous "overheating" problem.