Classic engines were designed around the petrol that filled the forecourts when the cars were new, four-star leaded, 98-octane, with the tetraethyl lead serving two purposes that have not been replaced in modern unleaded. The first was octane rating, holding off pre-detonation in higher-compression engines.
The second, and the one that catches owners out most often today, was valve-seat lubrication: the lead deposited a thin sacrificial layer on the exhaust valve seats during combustion that prevented the unhardened cast-iron seats from receding under sustained running. Modern UK petrol delivers neither, and the ethanol content of modern E10 unleaded introduces a third issue that did not exist in the leaded era. Castrol Classic Fuel Additives address each problem with a dedicated product, supplied alongside the established Classic Valvemaster range and the wider classic-fuel-system care products MGOC Spares stocks.
Valvemaster, Lead Replacement & Valve-Seat Protection
Classic Valvemaster is the established lead-replacement additive for classic-car owners, developed specifically for engines that have not been converted to hardened exhaust valve seats. Added to the petrol tank at the recommended dose rate, the additive recreates the sacrificial boundary layer the original tetraethyl lead deposited on the seat face, preventing recession on cast-iron unhardened seats under sustained running. Valvemaster Plus combines the valve-seat protection with an octane booster for owners running higher-compression engines that originally specified four-star petrol. Both products are stocked by MGOC Spares as a Castrol Classic Oils approved supplier, and full product detail and dose-rate recommendations are held at the sister site classicvalvemaster.co.uk.
For owners who have already converted to hardened valve seats during a cylinder-head overhaul, Valvemaster is no longer required for valve protection, but the octane content of UK pump petrol is still the consideration that decides whether Valvemaster Plus remains the right addition.
Etha-Guard, Ethanol Stabiliser for E10 Petrol
UK forecourts standardised on E10 unleaded in September 2021, petrol with up to 10% ethanol content by volume. Ethanol is hygroscopic, drawing moisture out of the atmosphere into the petrol tank; it is mildly corrosive to certain metals and elastomers found in older carburettor float bowls, fuel pumps, hoses and tank linings; and it degrades the calorific value of the fuel as water content rises during storage. Etha-Guard Plus, supplied with the Octimise Plus friction modifier, addresses all three: it stabilises E5 and E10 ethanol petrol, binds absorbed moisture into a non-corrosive form, neutralises the mild acidity of the ethanol-water reaction, and extends storage life. The additive is fully compatible with cars fitted with catalytic converters, making it the right product for younger classics and modern MGs that do not need valve-seat protection but do need to be protected against ethanol damage in storage.
Note that every product in the Valvemaster fuel additive range already contains Etha-Guard, for cars that need both valve-seat and ethanol protection, Valvemaster Plus is the single-bottle solution.
Application Across the Classic Fleet
For standard-engine MGB, MGA, Midget, MGC and the wider classic British fleet running on pump unleaded without hardened seats, Valvemaster Plus is the standard recommendation for every fill, valve-seat protection, octane boost and ethanol stabilisation in one. For cars with hardened seats fitted, Etha-Guard alone is enough where the car sees regular use, with the octane question answered by 97+ Premium Unleaded E5 rather than 95 E10 where the engine specification calls for it.
For cars laid up over winter or stored for months, treating the petrol with Etha-Guard before storage prevents the corrosion, gum formation and water separation that an untreated tank can develop over a long lay-up. The MGOC Spares technical team can advise on the right additive regime for a specific car, particularly where modifications, compression ratio changes or unusual use patterns affect the standard recommendation.