The Truth About Premium Fuel and Your Engine

Whenever I fill up at a forecourt, there is usually someone next to me staring at the row of pumps with a slightly puzzled expression. Regular unleaded at one price. Super unleaded at fifteen pence a litre more. A premium grade beside that at twenty-five pence more, with names that sound like they belong in a perfume catalogue. The question that hangs in the air is always the same. Is it worth it. Does it actually do anything. Or is it, as someone said to me at the pumps last week with weary certainty, “just expensive petrol for people who do not know any better.”

The honest answer is more nuanced than either marketing departments or fuel-station cynics would have you believe. Premium fuel does something. The question is whether your specific car benefits from it. For most cars in everyday use, the answer is genuinely no, you are paying for something the engine cannot use. For a smaller set of cars, the answer is genuinely yes, and using regular fuel is mildly bad for the engine. Working out which category your car is in takes about thirty seconds of reading the fuel filler flap.

What “premium” actually means

The headline difference between regular and premium petrol is octane rating. Octane measures the fuel’s resistance to “knock,” which is a kind of uncontrolled combustion that happens when fuel ignites too early or too violently inside the cylinder. Higher octane fuel is more resistant to knock, which means it can be used in engines designed for higher compression or more aggressive ignition timing.

In the UK, regular unleaded is rated 95 RON (Research Octane Number). Super unleaded is 97 or 98 RON. The premium grades from Shell, BP, and Esso are typically 99 RON. Diesel works on a different rating (cetane), but the same principle applies: higher cetane diesels burn more cleanly and ignite more readily.

Alongside the octane rating, premium fuels usually contain a higher concentration of detergents and additives. These compounds help keep the engine clean: they reduce carbon build-up on intake valves, fuel injectors, and inside the combustion chamber. This is the part of premium fuel that benefits any engine, regardless of design, though to a measurable rather than dramatic extent.

How to know if your car actually needs premium

The single most reliable answer is on a sticker inside your fuel filler flap, or in the handbook. Look for one of three things:

• “Use unleaded 95 RON” or similar: your engine is designed for regular fuel. Premium will not damage it, but the engine cannot extract additional performance or economy from the higher octane. The only benefit is the detergent package, which is real but modest.
• “95 RON minimum, 97 RON recommended” or similar: your engine can use either, but is designed to perform best on premium. Regular fuel works but the engine management computer pulls back ignition timing slightly to protect against knock, which reduces power and fuel economy by a small amount.
• “Use unleaded 98 RON” or similar: your engine genuinely requires premium fuel. Running it on regular causes the engine to retard timing significantly, can trigger knock on hard acceleration, and over time may shorten engine life. This is almost always seen on high-performance engines, turbocharged or supercharged engines, and certain specific manufacturers (some BMW, Porsche, and AMG engines fall here).

If you cannot find a sticker or a handbook entry, check the manufacturer’s website for your specific model and engine. Do not assume; specifications vary between engines in the same model range, and the diesel handbook entry does not apply to the petrol version of the same car.

The performance case

For cars that are designed for higher octane, the difference is genuinely measurable. Several independent tests have shown that engines designed for premium fuel can produce 3 to 7 percent more power and slightly better fuel economy when run on premium versus 95 RON. On a long journey, the fuel economy improvement can offset a meaningful portion of the price difference, though rarely all of it.

For cars not designed for premium (the majority of family cars sold in the UK), the same tests typically show no measurable performance gain. The engine management cannot take advantage of the higher octane, so the fuel essentially burns slightly more cleanly and that is all.

The cleanliness argument

This is where the marketing tends to make the most of itself. Premium fuels do contain higher levels of detergent additives, and these additives can genuinely reduce carbon build-up on intake valves and injectors. Research from various automotive bodies has shown measurable differences in deposits between engines run on premium versus regular over tens of thousands of miles.

For most owners, however, the practical effect is small. Engines built since the early 2000s have generally good fuel management and tolerant carbon levels. A tank of premium every few months, or occasional bottles of fuel system cleaner added to regular petrol, achieves much the same result at a fraction of the cost. Some owners use premium for one tank in four as a kind of internal flush; whether this is actually beneficial is debated but it is not harmful.

What about diesel?

Premium diesel grades are common too. They typically have slightly higher cetane (better ignition), more detergents, and sometimes higher lubricity. The case for premium diesel is generally stronger than for petrol because diesel injectors run at extreme pressures and benefit from the added lubricity, and modern diesel emission systems benefit from cleaner combustion.

For a daily-driven diesel, occasional tanks of premium diesel are a reasonable compromise. For high-mileage diesels (especially older common-rail engines with high-pressure injection), using premium diesel consistently can extend injector life noticeably. The cost difference is usually less of a percentage of the total fuel bill than for petrol.

The fuel economy maths

For a car that does not need premium, the maths is straightforward. Premium fuel costs roughly 10 to 15 percent more per litre. Without a measurable economy or performance benefit, you are simply spending 10 to 15 percent more for fuel that the engine treats much the same. Over a year of typical driving, that is several hundred pounds with no return.

For a car that does need premium, the maths is more interesting. The 5 to 8 percent better fuel economy that some engines achieve on premium offsets a portion of the higher cost, but rarely all of it. You may pay a net 5 percent more per mile. The trade-off is the engine performing as designed and (likely) lasting longer in its intended state of tune.

What about additives in a bottle?

If your engine does not need premium fuel but you want some of the cleaning benefit, an occasional bottle of fuel system cleaner from the parts shop is the most cost-effective option. Major brands (Redex, Wynn’s, BG, Liqui Moly) all produce good products, and a single bottle (around six to ten pounds) added to a tank of regular petrol gives the engine essentially the same detergent boost as several tanks of premium.

Once every three or four months, a bottle of cleaner added to a regular tank of petrol gives you the cleaning benefit at a fraction of the cost. Some owners do this more often, some less. There is no harm in occasional use, but adding cleaner to every tank is unnecessary and wasteful.

What the marketing does not say

Premium fuels are real products with real differences. The advertising, however, tends to imply that any car runs better on premium, which is not true for engines that cannot use the extra octane. The cleaning claims, while genuine, are often overstated; a well-maintained modern engine, run on decent regular fuel from a busy filling station, does not develop dramatic deposits in normal use.

The case for premium is best made when the engine actually needs it. Marketing aimed at owners whose cars do not need it is, broadly, a way to sell more profitable fuel without making any false claims; the fuel really is higher octane and really does contain more detergents. The implication that this matters in every car is what blurs the line. The same kind of small daily attention that we covered in the Sunday morning routine matters far more to engine longevity than the choice of fuel grade for most cars.

The simple recommendation

Look at your fuel filler flap. If it says 95 RON, run regular. If it says 97 or 98 RON minimum, run premium. If it says 97 or 98 RON recommended, do the maths on your own driving and decide. There is no scandal here, just a more nuanced answer than the pumps would suggest.