When I bought my '98 S70 (base, manual shift) , I used mid-level gasoline (89 octane) for the three years I drove it. Even though the manual called for premium gas, I experimented with 87 and 91 octane and the car seemed to automatically adjust the timing and run just fine. By that, I mean, I really couldn't tell the difference but I continued to use 89o.

Last October I bought my present '00 S70 GLT/SE (automatic) and, like the '98, it was delivered with 91o in the tank and the manual recommended 91o. This time, I used 91o for almost 4 months in order to get the full feel of its performance on streets and roads I travel daily. Then, I started using 87o thinking I would be able to feel the drop in performance in my normal driving after such a long time using 91o. If I felt the drop, I had planned to then try 89o for a while.

BUT, I felt no difference whatever in the performance. I decided to give it more time and the 87o usage stretched out to over a month. This light turbo adjusted to the lower octane like my base model did three years earlier. (My wife's '01 C320 Mercedes gives all sort of warnings about using anything lower than 91o. You'd think the car will melt in the driveway if you use a tank of non-premium fuel.)

With gas prices punching through the roof, I have stayed with 87o. I would use a higher octane if I could even say "I THINK I feel a drop in power or some other difference, but I can't. Has anyone else had this experience?