The good - In the early 70's, during the first gasoline shortage, one of the sports car magazines thried mixing fuel - They found that half high test and half regular gave a mixture that performed almost as well as all high test.

The bad - When the knock sensor senses pre-ignition, retarding the timing does NOT cure the problem, as the mixture is firing before the ignition fires anyway - many, if not most systems add extra fuel to cool the combustion chambers, then retarding the timing does some good. This process uses extra fuel and causes poor performance.

The ugly - If you look at gasoline prices, the relationship between grades is different from years ago. A couple of MPG is worth the extra money.

And, your engine will love you.

BTW - older engines are more prone to knock than new ones - tests on relatively new cars may not be representative of older ones.