[trube78]

1. If you have performance software you will notice it in HP and MPG. The new software gets some of its gains by decreasing tolerances to allow for a specific octane rating (among other things). Factory software allows for a larger range of octane ratings although engine performance data (HP, torque, and MPG) is taken with higher octane rating i.e. the recommended octane.
2. I tried midgrade vs high test (never considered low test) in a 3.0 Z3 and an MDX and found.
a. In the city negligible gas mileage change about 1/2 MPG. Power may have been decreased but it was not really noticable (IMO). Others say it is but I found it unusual to be pushing the car in the city that I might notice.
b. On the highway, lost about 2 MPG with Midgrade. Power loss was noticable in the mountains but not so much in the flats. Did not compare 0-60 times as that required a good place/way to measure and more than a few runs to get enough data. I even made some identical long trips 300+ miles with both fuels and the results always showed a loss of about 2 MPG.
3. As far as engine damage, your mechanic is right the computer will sense knocking and other problems historically (in older cars) associated with lower grade fuel and compensate. However as I stated above MPG, HP, and Torque will be affected.
4. Finally, for highway driving if you are getting 20 MPG with midgrade and 22 MPG with premium. At current prices (i.e midgrade at about 2.50 per gallon). High test would need to cost over 25 cents per gallon more to make midgrade a financial bargain.
Hope this makes sense.
ATB,
Tom