Typically the manual will tell you to run low test (87 octane) and nothing else. I don't personally have an S80 but I've found that with most volvo's the best thing to do short of running high test all the time is to run low or mid grade most of the time and run a tank of high test (93 or higher) every 8 or nine tanks. This serves to clean out any gunk left by the lower octane fuels. If you can stick to mid grade all the time then go every 12 tanks or so and run high test. Running low all the time is the primary reason our cars fail emmission tests due to unburnt deposits on the valves, while running nothing but high test is ok but expensive.