Here in Australia, we have 3 common octane ratings available. 91, 95 and 98. I own a Holden Kingswood from 1978 with the 202 inline 6 cylinder engine which was obviously designed to run on leaded fuel. Please keep in mind things might be different here in Australia with fuel etc. than America. My question is (I am also using a lead replacement additive) which fuel octane would be most suitable? I am not sure how high octane the fuel would be back when the car was made in 1978.

For any Americans reading this, Holden was a brand under General Motors so the engine likely shares many similarities with Chevy inline 6 engines of the time period. I know 91 is probably too low octane, I currently use 98 but just wondering if I should perhaps be using 95 instead alongside the lead replacement additive I am already using.