There are two different circumstances: Firstly "modern" engines with management systems that can be suitably mapped to use whatever fuel it is intended to use. Secondly older engines with carburetors and traditional distributors with a fixed advance curve. With these the type of fuel that is suitable will depend on how the engine was built (compression ratio, advance curve, etc.). I am guessing that most of this type of engine in Sevens will have been built with power in mind meaning that they will suffer on lower octane petrol. As already noted V power will meet the octane requirement and possibly also give a small power boost.