RE: Octane Ratings : Old vs. New :
When our beloved MoPars were new , octane was rated via the
Motor Octane Number (MON) ; this is the higher number .
Europeans rated their octane based upon the Research Octane Number
( RON ) ; this was the lower number .
I'm not certain when the ratings were combined to arrive at the "A.K.I."
( Anti-Knock Index ) , commonly known as the "R+M/2" rating
( Research + Motor , divided by 2 ) , but all gasolines have been rated this way for at least 20-25 years .
Basically , the old rating of 100 octane gives up ~5 points to the new rating , henceforth it's now 95 octane , ad seq.
So , if your Duster's owner's manual is calling for "regular @ 91 octane minimum" , in today's ratings , that would be 86 octane .
Truly , our oldies -- in my experiences -- do run better on higher octane gasses ; I burn 91 ( highest pump gas rating in good ol' Cali ) in my '72 Coronet with a stock 318 and maxed-out timing advance.
Also , by 1972 , Chrysler started hardening the valve seats on all of their motors . Even though "Ethyl" was still abundant , low-lead and unleaded were becoming more common ( esp in Ca. ) , as there was a targeted phase-out of super leaded gasses by c.1975
( I still remember when Chevron was removing its "Custom Supreme" 105 octane pumps and replacing them with shitty unleaded , though this was probably exclusive to Ca. :mumum: ).