Fuels
What comes out of a forecourt pump is a specified product, made to a British and European standard, with a defined renewable content. This page sets out what those products are.
Petrol
Standard unleaded is sold at 95 research octane. Super grades are sold at 97 or 98. Octane is a measure of resistance to pre-ignition, not of energy content: a higher octane fuel does not contain more energy, and in an engine not designed to exploit it, it produces no benefit worth paying for. In an engine that is, it allows the ignition timing to be advanced and does.
Ethanol content
Standard petrol in the United Kingdom contains up to ten per cent bioethanol, which is why it is labelled E10. Super grades are generally E5, up to five per cent. The distinction matters for older vehicles and for small engines, where ethanol can attack certain fuel system materials and where the fuel is more prone to absorbing water while stored.
Ethanol also carries slightly less energy per litre than petrol, so consumption on E10 is marginally higher than on E5. The difference is small and is often mistaken for something larger.
Diesel
Forecourt diesel is duty-paid road diesel, undyed. It contains a biodiesel component, fatty acid methyl ester, up to the permitted proportion. Diesel is more sensitive to cold than petrol: waxes come out of solution as temperature falls, and winter grades are formulated to keep the fuel flowing through a filter on a cold morning.
| Grade | Typical octane / grade | Renewable content |
|---|---|---|
| Unleaded | 95 RON | Up to 10% bioethanol (E10) |
| Super unleaded | 97–98 RON | Up to 5% bioethanol (E5) |
| Diesel | Cetane to standard | Biodiesel component (FAME) |
| Premium diesel | Cetane to standard | As diesel, with additive package |
Additives
Premium grades carry detergent and lubricity additive packages intended to keep injectors and valves clean and to protect fuel system components. The effects are real but cumulative and modest; they are not a repair for an engine that already has a fault.
Misfuelling
Putting petrol into a diesel vehicle is the common and serious version, because petrol is a poor lubricant and a modern high-pressure diesel injection system relies on the fuel to lubricate it. Damage can occur within a very short distance. The correct response is not to start the engine at all and to have the vehicle drained.
Storage life
Petrol degrades faster than diesel; both degrade. Fuel left in a tank over a long period loses volatility, absorbs water and, in the case of diesel, can support microbial growth at the water interface. Equipment used seasonally is better stored with a full tank than an empty one.