Modern pumps were never designed for a 1930s-to-’50s sports car. Today's petrol is unleaded and laced with ethanol; today's oils carry a fraction of the zinc these engines were built around. A whole industry of bottles promises to bridge the gap — some genuinely useful, some pure marketing. Here is what each problem actually is, and what a T-Series MG really needs.
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.
This matters more for a T-Series than for many classics, because the XPAG and XPEG engines (and the TA's MPJG) have a cast-iron cylinder head with soft, integral valve seats — exactly the metallurgy the Federation of British Historic Vehicle Clubs identified as most at risk. It is not a reason to panic — these cars are usually driven gently, and the engine's modest compression keeps the risk down — but sustained hard use, hills and fast road work are where recession bites.
There are two honest answers. The first is a valve-recession additive in every tankful: the FBHVC tested and endorsed several, and the ones that work are metallic — potassium, sodium, phosphorus or manganese based. Names you will see include Castrol Valvemaster, Millers VSPe and Redex. Not every bottle passed the testing, so an endorsed one is the safe choice. The second — and permanent — answer is an unleaded cylinder-head conversion: hardened valve seats machined into the head so it needs no additive at all. This is common on T-Series engines, and a well-sorted car may already have it done. The featured WSU 141 runs a ported, unleaded head, for example. If your head has been converted, you can forget lead additives entirely.
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 a T-Series that means the SU carburettors' seals and the fuel lines — perished diaphragms, swollen or hardened hose, and classically the cork float and its washer. 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 XPAG, like every pushrod engine of its era, runs a flat cam-follower valvetrain — 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 in an XPAG quietly accelerates cam and follower wear.
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 XPAG is a low-compression engine — well under 8:1 — so today's ordinary unleaded, let alone ‘super’, gives it far more octane than it will ever need. Standalone octane boosters do nothing useful for a T-Series 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 a T-Series MG 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 an unleaded head (hardened 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 SU seals for ethanol-resistant ones.
- Skip the standalone octane boosters and miracle cure-alls — a low-compression XPAG has no use for them. No bottle replaces the real fixes: hardened seats, an ethanol-proof fuel system and the right oil.
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.