Ethanol gets blamed for everything, when it is only occasionally the problem.
As the what is the correct fuel... there is no hard and fast answer, despite what anyone says.
Your car is designed to run happily on 91, IIRC. But it is now 32 years old and that brings a bunch of variables into the equation that NOBODY can reasonably predict.
Here’s what I do with a new-to-me car: Keep running whatever the previous owner was using. Keep track of the economy, and how well it runs. Do this for a number of tanks until you are comfortable that I know the car and how it behaves in various situations.
Then swap to the next step better (or next step worse) fuel and repeat. Don’t worry about the first tank or two while the ECU learns the new fuel. But I start paying attention to the third and subsequent tanks.
This will give me a clear idea of whether you are heading in the right direction or not. Repeat until you are satisfied.
For example:
My old S40 T4 would run happily enough on 91, but 95 improved the fuel economy significantly - it was actually cheaper to run on 95 because it used so much less AND performance was noticeably better.
On 98, it got the same economy as 95, but ran better again - in particular, it would come onto boost far more easily.
So in that car, the choice was 95 vs 98 depending on how broke I was...
Alternatively, the old Falcon ute ran the same and got the same fuel economy regardless of what we put into it, so it was fed a diet of E10.