My 68 Merc's 390 before the overhaul wanted only 93 with 10.5 to 1 and the stock cam. After the overhaul, 10.25 to 1 and a Crane 272 degree Energizer cam it was fine with 89 and 12* BTDC timing. My 331 wants only 93 with 10.4 to 1, Ford Z303 cam and Canfield heads. My 89 Ranger's roller 5.0 (ported E7's, Ford F4TE roller cam, 9 to 1 ratio pings ever so lightly on 87 and 12* timing, never a sound with 89. The last 67 Stang I had wanted only 93 with a rebuilt 302, 9.5 to 1 ratio, C6OE heads, RV cam (don't know the specs but going by the sound it made idling), cast flat tops .020 down the bore at TDC. I'm with the others, run as little octane as you can with the timing advanced and keep it from pinging. Retard the timing and you lose power and the mileage suffers too. The economics work out in favor of midgrade sometimes over regular, the extra ten cents per gallon often get you a few more mpg than 87 and the cost per mile is less.