I finally (it took a while to find the perfect search query) found something that I find definitive regarding the fact that my 72's Owner's Manual requires 91 octane (Premium today). Apparently, the gas is basically the same, but they changed the formula that determined octane.
Found here:
http://forum.studebakerdriversclub.com/archive/index.php?t-44301.htmlQuoted response:
"Keep this in mind when thinking about octane levels in gasoline. Prior to 1972 in the US the posted pump octane was the Research Octane Number (RON), this number is still used in Australia and Europe. The current (Post 1972) octane posed on the gasoline pumps in the AKI (Anti-knock Index) or some times shown as (R+M/2) which means it is the average of the old RON value and the Motor Octane Number (MON).
As a rule of thumb the difference is about 4 octane numbers (exact calculations would be dependent on additives, ethanol content and other variables) so for the common grades now sold in the US I will list new and old methods.
New: AKI (R+M/2) vs. Old: RON
87 ____ 91
89 ____ 93
91 ____ 95
93 ____ 97"
Apparently, that's also why thousands of BMW owners are still scratching their heads and running race gas or octane boosters.

lol
Just wanted to pass this along, now that I found it. I ran midgrade (89) and/or premium (93 here), but recently have been running regular due to budget. It was relieving to know I wasn't hurting it.

Regarding the source, I figure if it's good enough for a Stude, it's good enough for a Pinto.
