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Why the Handbook Beats the Badge

Walk into any motor factor and you will be confronted by a wall of brightly coloured bottles, each promising better protection, smoother running and a longer life for your engine. The temptation is to reach for the one with the most impressive label or the name you recognise from the racing pages. Resist it. For a classic car, the single most useful document you own is the owner's handbook, and the oil you choose should be dictated by the numbers printed in it, not by marketing.

The grade and specification tell you everything that matters. A 20W-50 mineral oil is a very different proposition from a 5W-30 synthetic, and an engine designed in 1968 for the former will not thank you for the latter. Viscosity describes how thick the oil is when cold and when hot; the specification codes that follow describe the quality and additive package. Get both right and your engine will be happy for years. Get them wrong and you may find yourself with tappet rattle, oil leaks, or worse.

Understanding Oil Grades

The numbers on the bottle follow a system laid down by the Society of Automotive Engineers, and they are not as complicated as they look. A grade such as 20W-50 means the oil behaves like a 20-weight when cold (the W stands for winter) and like a 50-weight when hot. The lower the first number, the more easily it flows on a frosty morning. The higher the second number, the better it maintains its film strength at operating temperature.

• Monograde oils such as SAE 30 or SAE 40 were common in older engines and still suit some very early designs, particularly in warm weather.
• Multigrade oils like 10W-40, 15W-40 and 20W-50 cover a wider temperature range and are the practical choice for most classics used regularly.
• Lower cold numbers (0W, 5W, 10W) flow more freely when cold but are often too thin for older bearing clearances and flat-tappet camshafts.
• Higher hot numbers (40, 50, 60) hold pressure better in worn or hard-worked engines, which is why 20W-50 remains so popular with classic owners.

A common mistake is assuming that thinner oil is always more modern and therefore better. In an engine with generous clearances and a design that predates multi-grade synthetics, a thin oil can struggle to maintain oil pressure at idle and may not protect the camshaft adequately on start-up.

Specifications Matter as Much as Viscosity

Viscosity is only half the story. The specification codes that follow – things like API, ACEA or the older manufacturer-specific requirements – describe how well the oil resists wear, sludge, corrosion and thermal breakdown. These codes are updated over time, and a modern oil meeting the latest standard may actually be a poor choice for an older engine.

The most important consideration for many classics is zinc and phosphorus content, often referred to as ZDDP. These additives provide excellent anti-wear protection for flat-tappet camshafts and lifters, which were standard fitment on virtually every engine built before the 1990s. Modern oils formulated to protect catalytic converters and emissions systems have progressively reduced ZDDP levels, and running one in a classic can accelerate camshaft wear, particularly during the critical first few minutes after a cold start.

If your handbook specifies an older API rating such as SE, SF or SG, take that seriously. A classic-friendly oil that meets those older specifications but uses modern base stocks is often the ideal answer.

Mineral, Semi-Synthetic or Fully Synthetic?

Base oil type is the next decision. Mineral oils are refined directly from crude and suit engines that were designed around them, especially those with cork gaskets and rope seals that can weep when fed a very thin synthetic. Semi-synthetics blend mineral and synthetic base stocks for a sensible middle ground, offering better high-temperature stability without being so slippery that older seals start to leak.

Fully synthetic oils offer the best protection against wear and thermal breakdown, and they resist sludging far better than mineral oils. They are an excellent choice for a rebuilt engine with modern seals and tight tolerances, or for a classic that sees hard use, track days or long motorway journeys in hot weather. They are usually a poor match, however, for a tired, high-mileage engine that has spent decades on mineral oil – the detergents can loosen deposits and the thinner film may find its way past worn rings and seals.

Reading the Handbook and Checking the Dipstick

Before you buy anything, find the handbook and note the recommended viscosity, the specification code, and any notes about climate or heavy-duty use. If the handbook is missing, a workshop manual or a specialist club technical adviser will have the figures. Write them down and take them with you.

Once you have the right oil in the sump, check it properly. Park on level ground, let the engine sit for a few minutes so the oil drains back, pull the dipstick, wipe it, reinsert it fully, then read the level. Top up in small amounts and give the oil time to settle before checking again. Overfilling is just as harmful as running low, as it can aerate the oil and force it past seals.

• Check the level weekly, and always before a long run.
• Change the oil and filter at least annually, regardless of mileage.
• Change more often if the car does many short journeys or sits for long periods.
• Warm the engine gently before draining, and dispose of used oil at a proper recycling point.

Oil is cheap compared with engine work, and the right grade from a sensible manufacturer will serve you far better than the most expensive bottle of the wrong stuff. Read the handbook, match the numbers, and your classic will reward you with many more miles of happy motoring.