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My opinion: I own a 97 850-R with 55000 miles on it. I bought it new in Boulder, Co in January 1997. I asked the dealer what octane to run it at, and they said to run it on whatever makes it run well. I run 85 octane in it, which runs fine, and honestly can't tell the difference in performance from 87 or 91 octane. Never could with any car I have had. Our octane levels: 85,87,91. I don't use synthetic oil--regular oil works fine. I did have a volvo dealer mechanic say that it might be good to run on synthetic, but you still need to change it at the same interval as regular oil. He also said that regular oil is fine. Can anyone give me proof of the benefits of higher octane fuel? I am curious?
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