Unleaded, leaded, super unleaded — a short history

Leaded petrol left UK forecourts on 1 January 2000 — the finished tail of a 25-year phase-out that began when the catalytic converter was mandated on new petrol cars in 1986. The lead in "leaded" was tetraethyl lead, an anti-knock additive that also poisons people, soil, and most importantly catalysts.

Why it had to go

Two reasons, in roughly that order:

• Public health. Airborne lead from petrol exhaust accumulated in cities — particularly in children — throughout the twentieth century. The link to neurological damage was no longer arguable by the late 1970s.
• Catalytic converters. Lead permanently poisons the precious-metal catalyst that converts CO and NOx into less harmful products. Once the cat became standard, leaded fuel had to go or the cats stopped working.

So what is "super unleaded"?

Super unleaded — the green-handle E5 grade — is a higher-octane modern petrol with no lead in it. The name confuses people because it sounds like an upgrade fromunleaded. It isn't. It's a higher-octane variant of unleaded, with a slightly different blend and (depending on the brand) a better detergent additive.

What about classic cars?

Some pre-1996 cars were designed for the "cushioning" effect lead had on valve seats. Without it, soft cast-iron valve seats can recede over many thousands of miles. Two options:

• Lead-replacement additive. Bought from a classic-car parts supplier, dosed at every fill. Cheap and effective for occasional-use vehicles.
• Hardened valve-seat conversion. A one-off machining job done to the cylinder head. Pricier upfront but permanent — no additive needed afterwards.

The Federation of British Historic Vehicle Clubs maintains a list of which engines need which approach. If your car is older than your driving licence, that's where to look.