I have 3 Volvos; a 96' 850 turbo with 140,000 miles, a 97' 960 with 125,000 miles (70,000 on new engine), and a 02' V70XC with 181,000 miles. Which of these cars if any require premium fuel? Most cars I've seen that require it state it on the cluster or fuel cap. These do not, but the owners manual suggests a 91 octane minimum. So is this just a suggestion? Will it hurt to start running premium on them now with the mileage that's on them, not knowing if they were ran on premium previously? Is the gas mileage, power gains, and benefits of running premium fuel worth it? With gas mileage in Illinois at $4.59 for premium and 4.29 for regular I need to know if this is gonna level out the initial cost by providing better mileage and benefits.