E10 Petrol and the Triumph Stag

Section 1

E10 vs E5, briefly

The "E" number on the pump is the maximum percentage of ethanol blended into the petrol. E10 contains up to 10% ethanol and is 95 octane; it became the UK's standard unleaded in September 2021. E5 contains up to 5% ethanol and is retained as the higher-octane "super" or "premium unleaded" grade, typically 97 or 99 octane, still sold at most UK forecourts. E5 usually costs around 15–18 pence per litre more than E10.

When the switch happened, the government estimated that somewhere in the region of 600,000–700,000 older UK vehicles were not compatible with E10 — a large proportion of them classics. The Triumph Stag sits firmly in that group.

Section 2

Why ethanol is a problem for a Stag

Ethanol behaves quite differently from pure petrol, in several ways that all matter to a 1970s car:

• It is hygroscopic. Ethanol absorbs water from the atmosphere, which encourages internal corrosion of tanks, lines and carburettor bodies. In a car left standing, the fuel can eventually separate — the water and ethanol dropping out of the petrol into a corrosive layer at the bottom of the tank.
• It is a solvent. Ethanol attacks and degrades the older rubber hoses, seals, diaphragms, cork floats and gasket materials found in a 1970s fuel system. Parts that lasted for decades on straight petrol can soften, swell or perish surprisingly quickly.
• It corrodes some metals. Certain lower-grade zinc, aluminium and brass alloys used in period carburettors and tank fittings are more vulnerable to ethanol-blend fuels than to pure petrol.
• It changes the mixture and volatility. Ethanol leans the air/fuel mixture and alters how the fuel behaves at different temperatures, which can worsen cold starting and — in hot weather — encourage hot running and fuel vaporisation.
• It cleans — sometimes too well. Ethanol can dissolve decades of varnish and sediment inside an older tank, then push that debris straight into the filter and carburettors, where it causes blockages.

The Stag's twin Zenith-Stromberg carburettors, its period-spec rubber fuel lines and its steel tank are all exposed to every one of those effects. That is why E5 is the sensible choice, and why owners who run E10 by mistake often report soft or perished hoses, sticky floats and gummed-up carburettors within a season or two.

Section 3

What to actually do

Three straightforward habits cover most of the risk:

1. Fuel it with E5 super unleaded (97+ octane). Make it the Stag's normal diet. Do not switch to E10 to save money — the potential damage far outweighs the per-litre saving.
2. Add an ethanol-protection / fuel-stabiliser additive. Products such as Millers VSPe, Sta-Bil and similar are designed to protect against ethanol corrosion, absorb small amounts of water, and (in some cases) add valve-seat protection and a modest octane boost. This is especially valuable if the car is used infrequently or laid up between drives.
3. Upgrade vulnerable parts over time. When fuel lines, seals or gaskets come up for replacement anyway, fit modern ethanol-resistant equivalents — for example rubber hose rated to SAE J30 R9, and modern ethanol-tolerant carburettor gasket sets. Do not wait for a leak to force the issue in a petrol-soaked engine bay.

For long storage, do not leave ordinary pump fuel sitting in the system. Use E5 stabilised with an appropriate additive, and cross-reference the Storing Your Classic Car guide for winter and summer lay-up. Where a car is particularly cherished, or will sit for a long time, specialist low- or zero-ethanol "classic" fuels such as SUSTAIN Classic or Aspen (an alkylate petrol) are an option, at a considerable price premium. Cheap ethanol-content test kits are also widely available if you want to check what your local forecourt is actually dispensing.

Section 4

The bigger picture — why this matters now

E5's long-term future is not guaranteed. The UK government has committed to keeping E5 available only as the "protection grade" for the foreseeable future, with a formal review scheduled for 2026. Low-ethanol and ethanol-free alternatives aimed at the classic market are emerging, and the picture is likely to keep evolving. It is a live issue worth keeping an eye on as a classic owner.

None of this is an argument against E10 in principle. E10 exists for legitimate CO₂-reduction reasons and works well in the modern cars it was designed for. It is simply not the right fuel for a 1970s Triumph Stag, and treating it as such is a reasonable, proportionate response for an owner who wants to keep the car reliable and original.

Useful products

Product references

Neutral, product-agnostic references. Owner-recommended options can be linked here in future.

Common questions

FAQ

Can a Triumph Stag use E10 petrol?

No. The Triumph Stag is a 1970s carburettor car and is not listed as E10-compatible. Ethanol attacks the period rubber hoses, seals and gasket materials in the fuel system and can corrode metals in the carburettors and tank. Use E5 'super unleaded' (97+ octane) instead. This is general guidance — confirm compatibility for your own car with a marque specialist.

What fuel should I put in a Triumph Stag?

E5 super/premium unleaded, typically 97 or 99 octane, available at most UK forecourts. Adding an ethanol-protection fuel stabiliser (Millers VSPe, Sta-Bil and similar) is sensible, especially if the car is used infrequently or laid up between drives. For long storage or very cherished cars, low-ethanol or ethanol-free 'classic' fuels such as SUSTAIN Classic or Aspen are an option.

Is E5 still available in the UK?

Yes. Since the switch to E10 as the standard unleaded in September 2021, E5 has been retained as the higher-octane 'super' or 'premium unleaded' grade at most forecourts. It usually costs around 15–18p per litre more than E10. The government has committed to keeping E5 as the 'protection grade' for the foreseeable future, with a review scheduled for 2026.

Does ethanol damage classic car carburettors?

Ethanol can attack older carburettor internals — rubber diaphragms, cork floats, gaskets, seals — and can promote corrosion of some soft metals. It is also hygroscopic, so it absorbs water from the atmosphere, which encourages internal corrosion of the carburettor bodies and fuel lines. The Stag's twin Zenith-Stromberg carburettors are vulnerable, which is one reason E5 is preferred.

How do I protect my classic from E10?

Three steps: use E5 super unleaded rather than E10; add an ethanol-protection / fuel-stabiliser additive at every fill (or at least regularly); and upgrade vulnerable fuel-system parts — hoses to modern ethanol-resistant type (such as SAE J30 R9), plus modern seals and gaskets — as opportunities arise during servicing. For long lay-up, do not leave ordinary pump fuel sitting in the system.