I run 93 Octane, which is highest I can get in the Houston area. So far I haven't used any additives, but I haven't driven the GTO in hot summer weather either (been gone all summer).
I use ExxonMobil gas exclusively, mainly because I work for ExxonMobil, but also because the gas seems to last longer than the Valero gas I used to buy (closest place to the house). My Corvette is particularly sensitive to old gas, and the Valero stuff would make it start to run poorly after just a couple of weeks. I also started putting StaBil in the tanks if I'm going to be gone or not driving the cars for a while for whatever reason. They (StaBil) make a new formulation that is supposed to help with the higher alcohol content fuels that are being sold these days.
'68 GTO, 400/TH400, AC/PS/PB/PW, hideaways, Hurst dual gate
'72 442 'Vert, 350/TH350, AC/PS/PB/PW
'74 Corvette, 383/4-speed, 450 hp, restomod
'71 Javelin, 304/3-speed
'70 Cougar XR7 'Vert, 351W/FMX auto *SOLD*