A while ago my AU1 Falcon began running rough whenever I filled the tank with regular unleaded (91 octane I think). The problem was most noticeable when taking off. There was very noticeable shuddering and hesitation. Switching to premium unleaded (95 octane) fixed the problem. It seems strange that a car designed to run on regular unleaded petrol would all of a sudden run rough on it.

Does anyone have any ideas?

Here is a little story about my au tickford 6I fairlane.

Ran fine around Adelaide and country, but when towing a caravan and after first top up after entering Vic or NSW car would ping its head off on heavy acceleration or pulling big hills, the only thing that would help was using 98 octane fuel.

I did 3 trips and some 15,000 in three separate trips over 18 months with the same issues.

The only other issue was a slight miss on a cold morning if the car was parked on grass over night.

Then one day the coil failed, replaced coil and leads and while doing that also changed fuel filter.
Have done two more trips since and no more issues, not sure if it was the coil cross firing under heavy load or the old fuel filter couldn’t supply enough fuel and causing a lean state

Plugs and leads would be a good place to start