Shell said that, from a scientific perspective, most cars in the UK had the potential to save fuel by using petrol which contained a Friction Modifier, or diesel which contained a Combustion Improver, such as the Shell petrol and diesel FuelSave fuels. They said that was based on the ability of a Friction Modifier to reduce friction in petrol engines and of Combustion Improvers to reduce ignition delay in diesel engines. They said both of those phenomena were well known and accepted throughout the global fuel science and automotive engineering community
Shell said the substantiation showed that 60% of the cars tested achieved fuel savings of 2% or more, and that in their view the test findings indicated that the 2% benefit was potentially available to the majority of UK motorists. Shell said the unleaded fuel had also been tested in on-road vehicle testing in the UK. They said that over six of seven routes, cars fuelled with FuelSave used less fuel, in one instance measuring a saving of almost 3%. They said that there would always be uncontrolled variables in such tests that would make the outcome unpredictable, but the balance in favour of FuelSave was persuasive.
Link gives more info including Advertising Standards Agency response:
ASA Adjudication on Shell UK Ltd - Advertising Standards Authority