Power Beneath the Bonnet – The Hidden Science of Engine Oil

Engine oil has always been more than just a lubricant, it’s the lifeblood of every engine. From the days of crude blends of animal fats and mineral oils to today’s sophisticated formulations, the story of engine oil is one of constant development to meet the changing demands of engines. At Millers Oils, with our history stretching back to 1887, we’ve always adapted to these changes, creating lubricants that meet the needs of each new generation of engines. This means that whether you’re running the very latest technology or keeping a classic alive, there’s an oil formulation built to do the job.

In the earliest days of mechanisation, engineers turned to whatever lubricants they could find, often animal fats or vegetable oils to reduce friction between moving parts. These rudimentary lubricants worked well enough for slow-moving, low-powered machines. But with the invention of the internal combustion engine in the late 19th century, everything changed. Engines produced higher temperatures, greater pressures, and far more demanding operating conditions than any horse drawn carriage axle ever endured. Suddenly, lubrication wasn’t simply a convenience, it was a necessity. Without it, engines seized, components wore prematurely, and reliability suffered.

Early automotive oils were simple blends, largely unrefined and lacking the sophisticated chemistry we take for granted today. They darkened quickly, oxidised, and left sticky varnishes inside engines. Fast forward to today, and engine oils are complex mixtures of many different chemical components. They don’t just lubricate, they clean, cool, protect against corrosion, reduce wear, and in some cases even enhance fuel efficiency. Modern engines, whether a silky petrol or a torque rich diesel, simply couldn’t deliver their performance, efficiency, and durability without oils that meet the car’s specific technical standards.

At Millers Oils, we’ve been part of this evolution since 1887, just two years after Karl Benz built the first combustion engine and have continued to develop lubricants that adapt to the changing needs of vehicles across generations.

What Exactly is in an Engine Oil?

At first glance, engine oil looks like a uniform golden brown liquid. In reality, it’s a highly engineered product made up of a blend of many different chemical components.

A typical formulation consists of three major elements:

1. Base oils: the foundation, usually making up about three quarters of the oil.
2. Viscosity Modifier: long-chain polymers that help oil maintain optimal thickness across temperature extremes: thin enough to flow when cold, thick enough to protect when hot.
3. Additives: the chemistry that gives oil its performance characteristics.

It’s the interplay between these three that determines how well an oil performs in an engine.

Base Oils: The Foundation

Base oils provide the essential fluid properties of engine oil, and they come in several categories:

• Mineral Oils: Refined from crude oil, these oils tend to perform worse at low temperatures and break down quickly at high temperatures, less commonly used today in premium automotive engine oils, but are still seen in engine oils for older vehicles and select commercial vehicles.
• Synthetic Oils: Chemically engineered to deliver consistency, purity, and superior performance. These oils resist breakdown at high temperatures, flow more easily during cold starts, and keep internal components cleaner, all vital traits for modern engines, particularly those with turbochargers and advanced emissions systems. In other words, the “synthetic” base oils we think of are not just refined. When we talk about a fully synthetic engine oil, it is dependent on base oils that are purpose-built fluids, designed at a molecular level to be stable and effective in extreme conditions.
• Semi-Synthetic Oils: A blend of mineral and synthetic, offering a compromise between cost and performance. Semi-synthetics provide a step up in protection and stability compared to mineral oils. They resist breakdown for longer, keep engines cleaner, and offer improved performance in cold starts. While not as advanced as full synthetics, they are a practical choice for certain vehicles, particularly older models or those used in less demanding conditions, where full synthetic performance isn’t strictly required but mineral oil simply won’t do.

Performance and efficiency demands of modern engine oils are designed with synthetic base oils in mind, ensuring they can cope with both the performance demands and the efficiency targets that define today’s motoring experience.

The Secret Sauce: Additive Chemistry

If base oils are the bread, additives are the butter, the seasoning, and the vitamins all rolled into one. They transform a simple lubricant into a multifunctional performance fluid. Additives are the difference between an oil that simply prevents metal-on-metal contact, and one that actively works to protect, clean, and extend the life of an engine.

Modern engines place extreme demands on oil, high temperatures from turbochargers, rapid temperature swings from cold starts, stop-start driving in traffic, and the by-products of tight emissions controls. Additives are what allow oils to meet these challenges, maintaining performance and protection under conditions that would quickly overwhelm a plain base oil.

Key additives include:

• Detergents: Keep surfaces clean by neutralising acids and preventing deposits from forming on pistons and valves.
• Dispersants: Work hand-in-hand with detergents, suspending soot, dirt, and oxidation by-products in the oil so they can be flushed away during the next oil change.
• Anti-Wear Agents: Most famously ZDDP (zinc dialkyldithiophosphate), which forms a protective layer on metal surfaces, reducing wear when two parts make contact.
• Friction Modifiers: Reduce internal resistance, improving efficiency and in some cases even boosting fuel economy.
• Antioxidants: Prevent the oil from degrading due to heat and oxygen exposure, extending its useful life.
• Corrosion Inhibitors: Shield metal surfaces from acidic by-products of combustion.
• Foam Suppressants: Stop oil from frothing in high-speed operation, which would otherwise compromise its lubricating ability.

This complex blend ensures your car’s engine is protected whether you’re cruising gently on the motorway or pushing towards the red line on a twisty B-road.

The Viscosity Question: Why Thickness Matters

Viscosity is perhaps the most widely recognised characteristic of engine oil. You’ll have seen designations like 5W-30 or 0W-20 printed on oil bottles, and you might recognise these as part of the official oil specifications listed in your handbook. But what do these numbers actually mean?

In simple terms, viscosity refers to how resistant a fluid is to flow, or, put another way, how “thick” or “thin” it is. But thickness isn’t static; it changes with temperature. Honey flows slowly on a cold day but runs freely when warmed. Engine oils behave in much the same way, and viscosity grades are designed to show how an oil will perform in real-world conditions.

The numbers represent how the oil flows at both cold-start and normal operating temperatures, as well as how resistant it is to shearing forces inside the engine, for example, between piston rings and cylinder liners. This is crucial: using the correct viscosity ensures that oil flows quickly enough to protect on cold starts, yet remains robust enough to guard against wear and maintain pressure when the engine is hot. Manufacturers have specific standards that oils must meet to ensure optimal performance and engine longevity. Oils meeting these specifications undergo rigorous testing, including assessments conducted by specialist laboratories like Millers Oils, which evaluate performance under a range of temperatures, pressures, and wear conditions.

Even within the same viscosity grade, there are many types of engine oils available. This is because viscosity alone doesn’t define an oil’s performance. Oils are carefully formulated with different base oils and additive packages to balance characteristics such as oxidation resistance, deposit control, shear stability, and friction modification. The right combination depends on the engine’s demands, for instance, a high performance turbocharged engine requires an oil with excellent thermal stability, whereas a lightly loaded commuter engine may prioritise fuel efficiency.

Ultimately, selecting the correct oil isn’t just about matching a number on a bottle. It’s about using a product tested and certified to meet your car’s exacting standards and designed to perform under the specific conditions your engine experiences.