Leaded super was traditionally 98 Research Octane Number. That octane level became progressively harder to achieve as legislation gradually reduced the amount of Tetra Ethyl Lead that was allowed in fuel. In the end there was not very much difference between the "base stock" super before the lead was added, and 91 octane unleaded. The cars of the late days of the leaded fuel era tended to push the fuel pretty hard: Holden V8s in particular, were subject to burst detonation when launching from a standing start, and some trace detonation when they got a bit hot, on 98 RON leaded fuel. Trying to run one on unleaded 95 was a recipe for trouble, and not only because the octane was too low: they also relied on the lead to lubricate the valve seats. The unleaded V8 had induction hardened valve seats to control the problem.
Standard leaded was 89 Research Octane Number. Most Japanese cars of the period required 91 RON, so it was necessary to use super or shandy the two grades. (Some Japanese manufacturers had export models designed for the near-kerosene sold as fuel in South East Asia at the time.) The Japanese domestic fuel was always 91 RON as far as I know. If you've read about it being 89, I think you have read about it translated into American. The American practice is to rate the fuel on the average of the RON and the Motor Octane Number (determined in a variable compression ratio fuel-testing engine), and that is always lower than RON.
Part of the problem that arose when Australia switched to 98 leaded and 91 unleaded, was there was no place to get rid of the crummy base fuel stock that was previously used to make the 89 octane "standard" leaded. Fortunately by then very little 89 octane was being sold, since neither the Japanese nor the locally-made cars would run properly on it. Over the transition years, as the lead in leaded was reduced, and the 89 octane fuel went away, a considerably higher proportion of cracked hydrocarbons had to be used to produce the octane level required. That resulted in a lot of investment in the domestic refineries to increase cat cracking capacity, introduce hydro-forming, etc. The result was a reduction in the number of domestic oil refineries: the marginal ones had not done much if any cat cracking to begin with, and couldn't afford to start then.
