I have a N/A 04 5 Speed. On my many travels I logged mileage as well as other services. Since day 1, when I run the recommended 87, I get some octane knock. When I run my OBDII display on the passenger seat right next to me, I see the timing retard when I hear that knock. It goes from 42 BTC to 12 BTC ( at least I know the knock sensor is working ). The best mileage I got from 87 was 30 mpg.
Then on a lark, I tried 'plus' 89 ( .10 more per gallon ) , no more knock and no more retarded timing. My mileage increased and on the highway I consistently get over 30 mpg usually in the 33-34 mpg.
For me, and extra $1 or so at fillup is worth the extra 30 or more miles I get out of a tank of gas, and cheaper.
End of my $.02 and experiences.
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04XSPP(7 Subaru's since 1978 All 5 sp)ILuvSubaru's
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