I would bet that your engine plate say use 91 'research' octane, which was a requirement for a lot of GM engines in the 60's and 70's. The 89 octane gas that you buy today is actually rated as an average of 'research' octane and 'motor' octane. If you look on the pump, it should be labeled (r+m)/2. Since motor octane tends to be a few points lower than research octane the 89 'average' octane gas, is probably pretty close to 91 'research' octane.

Bottom line if you engine is not pinging or running on after you turn of the key you probably have enough octane.

You should probably be more concerned with having fresh, clean fuel.

Ok guys, I have checked Joe wheeler, Florence Harbor , Bay hill and Aqua Yacht Basin , they all have 89 and said I was the only one one the river that they knew of with 93. The major oil cos do not mix to 91 but you can get 89 and 93 on the truck and have 91 which they do not do. I have Diesel as does Joe wheeler. I am selling 93 for the same price or less than they are selling 89.