Use the highest octane unleaded you can find. Around here that's 93 or 94 octane. The 87 octane would require retarding the ignition timing and may cause damage. The early SOHC sixes are noisy enough to mask the sounds associated with "pinging", so its best to use the appropriate octane fuel for the car.
There were no differences in US Vs. European engines in 1966, but some of the models were made to run on the poor quality fuel that was once a problem in North Africa. So there were "low compression" engines built for these specific markets.
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