That explains why my gas mileage is going down. I was getting 25+ MPG on my 2001 S80 using 89 octane. Since the price of gas went up (and my S80 is 7 years old now) I changed to 87 octane and my MPG has gone down to 22+. I also have a check engine light and my mechanic thinks it is a faulty oxygen sensor but his software can't read the code. He said to take it to the dealer. Could it all be related to the quality of the fuel and to 87 octane?

O2 sensor replacement is not uncommon.

Octane is unlikely to be the problem, fuel quality is another issue and one can never tell.

ANyone else here gets their gas from citgo? I know all gas stations have regulations they have to pass and so on, but can it be said that any gas is better than the other? I honestly dont believe so...
I still find despite using 93 octane my mpg is suffering, im hoping that the tune up will help it out (new air, fuel, oil filter etc).. i really could do with the extra mileage.. and mine isnt even a t6