Fuel, oil and additives: running a Stag on modern pumps

The lead question — valve seat recession

Leaded petrol did two jobs: it raised octane, and it left a soft lead deposit that cushioned the exhaust valve seats. Take the lead away and, on an engine with the wrong metallurgy, the exhaust valve can momentarily micro-weld to its seat under heat and load and tear away a little metal every time it opens. The valve sinks into the head, the clearances close up, and in time it burns. This is valve seat recession, or VSR.

The Federation of British Historic Vehicle Clubs commissioned engine testing on this, and found VSR is a real risk on cast-iron heads and on aluminium heads fitted with soft valve-seat inserts — but mainly under sustained high load, speed and temperature. Gentle, low-compression pottering is far less exposed than hard motorway cruising.

Where does the Stag sit? The V8 has aluminium heads with pressed-in valve-seat inserts, so whether a given engine needs lead protection comes down to how hard those seats are. Plenty of Stags run happily on plain unleaded; plenty of owners add a valve-recession additive as cheap insurance, particularly for fast or sustained driving. The FBHVC endorsed several such additives after testing, and the ones that work are metallic — based on potassium, sodium, phosphorus or manganese. Names you will see on the shelf include Castrol Valvemaster, Millers VSPe and Redex; TetraBOOST is different again, being actual tetraethyl lead (real leaded fuel, but toxic and aimed at racing). Crucially, not every bottle passed the FBHVC testing, so an endorsed product is the safe choice.

The permanent answer is hardened valve seats, machined into the head when the top end is off. That is the once-and-done fix — and it is why a Stag whose heads have already been converted for unleaded needs no lead additive at all.

Ethanol — the E10 problem

Modern petrol carries ethanol: up to 5% in E5, and up to 10% in E10, which has been the UK's standard 95-octane grade since 2021. Ethanol is hygroscopic — it draws water out of the air — and, being a small, polar molecule, it seeps into and degrades the rubber, cork and fibre of a 1970s fuel system: hoses, seals, diaphragms and float needles. It can corrode some metals, and it carries less energy than petrol, so it slightly weakens the mixture.

The fixes are mostly mechanical: fit ethanol-resistant fuel hose and seals, favour E5 "super" petrol where you can, and — for a car that is laid up — treat the tank with an ethanol-stabiliser additive to hold off moisture and corrosion. We cover this in full on the dedicated E10 page.

Oil and zinc — the one most people miss

This one is about oil, not fuel, and it is the quiet engine-killer. The Stag V8, like almost every pre-1980s engine, has a sliding cam-follower valvetrain — the followers bear directly on the camshaft lobes under high contact pressure. Engines like this relied on a zinc-and-phosphorus anti-wear additive in the oil, ZDDP, which forms a sacrificial film that keeps metal off metal.

Here is the trap: because zinc and phosphorus poison catalytic converters, modern oils to API SM, SN and SP specification have had their ZDDP cut right down. That is fine for a modern roller-cam engine, but too little for an old sliding-follower one. Pour a cheap modern oil into a Stag and you accelerate exactly the cam-follower wear these engines are already prone to.

The fix is simple and cheap: use a classic-specific or high-ZDDP oil. A good 20W-50 formulated for classics carries the zinc these engines want — broadly in the 1,000–1,200 ppm region for street use — from the likes of Millers Classic, Castrol Classic, Valvoline VR1 or Penrite. It is not simply a case of "more zinc is better": the balance of zinc to detergent matters, which is why a properly formulated classic oil beats tipping a zinc bottle into modern oil. Confirm the grade against your workshop manual.

Octane boosters, and the rest

Octane only matters if an engine pinks — pre-ignites — which is a high-compression problem. The Stag V8 is a modest 8.8:1, and set up correctly on today's 97-plus "super" unleaded it rarely wants more octane. Most standalone octane boosters do little for a healthy Stag and are the most marketing-driven corner of the additive shelf. Upper-cylinder lubricants and friction modifiers — molybdenum-based products such as Molyslip, or Redex fuel-system cleaners — have their uses for cleaning and light lubrication, but none of them fixes valve seats, ethanol damage or cam wear, which are the three things that actually matter.

So what does a Stag actually need?

The honest bottom line:

The oil is the easy win everyone should take — run a proper classic high-ZDDP 20W-50, and you have cheap insurance against the cam-follower wear these engines are known for.

On fuel: if the heads have been converted to hardened seats, you need no lead additive; if not, an FBHVC-endorsed valve-recession additive is sensible insurance for anything beyond gentle pottering. Use E5 super where you can, fit ethanol-resistant hoses, and treat the tank if the car is laid up.

And skip the miracle cure-alls and standalone octane boosters. No bottle substitutes for the permanent fixes: hardened seats, an ethanol-proof fuel system, and the right oil.

A note on the brands above: we name them because they exist and are widely used, not because there is any commercial relationship here — there isn't one, and nothing on this site is a paid placement.