Most UK MGBs are standard high compression (HC) engines of 8.8:1. The above book states that fuels below 98 octane are not suitable. Only low compression engines can take fuel of octane up to 97 these are generally USA spec engines.
This means most unleaded fuels in the UK are not suitable as they are around the 95 octane level.
The fuel I use is the Tesco high octane at 99 octane. You can usually see the octane rating on the pump or the nozzle.
I think you will feel the difference between the fuel in way out performance
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