50/50 i think is considered optimal for all around purposes and is available in premixed containers everywhere. the 99 owners manual states to stay between 20 to 60%. lower than 20% antifreeze may lose anticorrosion effects (and lube for water pump) and over 60% loses cooling ability. also states to use whatever ratio is needed to protect to -33F and suggests refill/topping off when needed with 50/50.

personally, i keep it at 50/50 and use prestone, but may change to the more environmentally friendly type next time.

many go the extra step and use distilled water.