Coolant should really be changed every two years, regardless of mileage, or at the very least, every 60k. No such thing as a super coolant that will last the lifetime of the car. Use original Volvo coolant and distilled water, 50/50 mix. The same goes for the transmission fluid (every 20-25k) and brake fluid, even the power steering fluid (both every 30k). It's up to you whether or not you want to perform preventive maintenance, or just wait until things brake down.