Muddying the water slightly, contemporary sales literature for the MG Turbo (and the owners' handbook) advised drivers to use only 4* leaded petrol. This was because unleaded petrol was only available with an octane rating of 95 RON. The turbo engine was tuned for the higher 97 RON octane rating of leaded petrol, which improves combustion and reduces the chances of 'pinking' and running lean on boost: both very harmful to turbo engines. Modern high-octane unleaded petrol is a perfectly suitable substitute, but it had yet to become available on forecourts when the cars were current.