I have three vehicles(96-S-320, 97-SL320 and Volvo S-90) which call for high test (91 oct). I have been using mid-grade(87) since new with no ill efects at all. I've been told if you don't dog the vehicle, this is fine. As long as It does not ping or diesel at shut-off, no problem. With all the new electronic controls on todays cars, the computer will automatically adjust the timing to control knock, ect. These vehicles get the following mpg on road trips: 96-S320=27 mpg, 97-SL320=24 mpg and Volvo=23.5 mpg. The Volvo should do much better, but they do not use lockout in high gear, so every time you let up on the gas, it down-shifts(piss-pore xmission setup). Good luck