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 soft seats, the exhaust valve can micro-weld to its seat under heat and load and tear away a little metal every time it opens — valve seat recession, or VSR. The valve sinks into the head, the clearances close up, and in time it burns.
The Aceca was sold with three very different engines — the elderly AC two-litre six, the exotic Bristol two-litre, and, in the final cars, the Ford Zephyr 2.6 — but they share this vulnerability: all were designed around leaded fuel, and none was built with the hardened valve seats a modern engine takes for granted. The Federation of British Historic Vehicle Clubs identified exactly this metallurgy as at risk, chiefly under sustained load, speed and heat.
There are two honest answers, as with any engine from the leaded era. The first is a valve-recession additive in every tankful: the FBHVC tested and endorsed several, and the effective ones are metallic — potassium, sodium, phosphorus or manganese based. Castrol Valvemaster, Millers VSPe and Redex are names you will see, and an FBHVC-endorsed product is the safe choice, since not every bottle passed. The second — and permanent — answer is hardened valve seats machined into the head when it is off, after which no additive is needed.
One caveat peculiar to the Aceca: the Bristol engine in particular is rare and expensive to rebuild, so this is not a corner to cut on a Bristol-engined car. Protect it.
Ethanol — E5, E10 and old fuel systems
Modern petrol carries ethanol — up to 5% in E5, up to 10% in E10, the UK's standard 95-octane grade since 2021. Ethanol is hygroscopic (it pulls water out of the air) and, being a small, polar molecule, it seeps into and degrades the rubber, cork and fibre an old fuel system is full of. On an Aceca that means the carburettors and the fuel lines and seals — perished diaphragms, swollen or hardened hose, sticking float needles. It also corrodes some metals and slightly weakens the mixture.
The fixes are mostly mechanical: fit ethanol-resistant fuel hose and modern seal kits, favour E5 'super' petrol where you can, and treat the tank with an ethanol-stabiliser for a car laid up over winter. Ethanol does its worst to a car that sits with stale fuel in it.
Oil and zinc — the one most people miss
This one is about oil, not fuel, and it is the quiet engine-waster. The AC, Bristol and Ford engines are all of the flat cam-follower generation — the followers slide directly on the camshaft lobes under high contact pressure. Engines like this were built around a zinc-and-phosphorus anti-wear additive in the oil, ZDDP, which lays down a sacrificial film that keeps metal off metal.
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 — fine for a modern roller-cam engine, wrong for an old sliding-follower one. A cheap modern oil quietly accelerates cam and follower wear, and on the precious Bristol unit especially, the right oil is the cheapest insurance you can buy.
The fix is simple: run a classic-specific or high-ZDDP oil. A good 20W-50 formulated for classics — Millers Classic, Castrol Classic, Valvoline VR1, Penrite and the like — carries the zinc these engines want, broadly 1,000–1,200 ppm for street use. It is not 'more zinc is always better': the zinc-to-detergent balance matters, which is why a proper classic oil beats tipping a zinc bottle into a modern one. Check 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 three Aceca engines differ: the more highly-tuned Bristol is the one that most appreciates good fuel, but today's 97-plus 'super' unleaded is ample for all of them, and the lower-compression AC and Ford ask even less. Standalone octane boosters add little to a healthy engine and are the most marketing-driven corner of the shelf. Upper-cylinder lubricants and fuel-system cleaners (molybdenum products such as Molyslip, or Redex) have their place for cleaning and light lubrication, but none of them fixes valve seats, ethanol damage or cam wear — the three things that actually matter.
So what does an Aceca actually need?
The honest bottom line:
Run a proper classic high-ZDDP 20W-50 — the easy win, and cheap insurance for the cam and followers.
On fuel, decide once: either fit hardened valve seats and forget additives, or run an FBHVC-endorsed valve-recession additive in every tank. Use E5 super where you can, and renew perished fuel hose and seals for ethanol-resistant ones.
Skip the standalone octane boosters and miracle cure-alls. No bottle replaces the real fixes: hardened seats, an ethanol-proof fuel system and the right oil. And on a Bristol-engined car especially, get all of this right — the engine's rarity makes it the cheapest insurance you will ever buy.
We name the brands above 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.