What Are Engine Oil Additives?

By: Lisa Harper

When answering the question what are engine oil additives, we need to first understand what engine oil does.

Engine oil has two main purposes:

1. It lubricates all moving parts of a car’s engine to minimise friction between parts

2. It protects metallic surfaces from corrosion (rust)

To enable oil to effectively fulfil these two purposes it must be able to travel freely through a car’s engine and coat all the engines components. If engine oil is too thick it will struggle to travel freely enough and could even clog up the engine whilst if it is too thin it travels too quickly through the engine and doesn’t coat all the engine parts.

The problem in car engines is the change of temperature, from a relatively cold environment when you first start your car (especially on cold mornings) to a high-temperature environment when you have been driving for some time. This change in temperature can alter the thickness of your engine oil. When engine oil is cold it will be thicker and as it gets warmer it becomes thinner, this means oil may get too thin to be effective any longer.

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engine oil additives

Furthermore the heating of engine oil can cause engine oil to oxidise (combine with oxygen and lose hydrogen) which causes the formation of sludge, varnish, resin and corrosive acids which can negatively affect the engine and can cause corrosion (rusting) of engine components.

In order to combat both of these effects it is necessary to add additives to engine oil to help improve its ability to perform its two purposes.

What types of engine oil additives are there?

There are a number of common engine oil additives and they all have different functions;

  • Viscosity Index Improvers (VIIs) – As discussed engine oil needs to be the correct thickness (viscosity) to be effective. VIIs alter the temperature at which the viscosity of engine oil starts to change. This stabilises the engine oil and enables it to continue to be effective over a greater range of temperature difference.
  • The problem with VIIs is that they are prone to shearing (i.e. they break up) which prevents them working efficiently and is one of the reasons engine oil should be regularly changed.

  • Detergents – break down sludge and prevent the formation of deposits and corrosion (rust). Detergents are used up so after a time will no longer be present in engine oil, another reason engine oil should be regularly changed.
  • Dispersants – absorb solid contaminants within the oil and help to delay the formation of sludge, this helps prevent contaminants from damaging the engine.
  • Antioxidants – help to slow down the process of oxidisation and thus the formation of deposits. This helps keep the engine stay cleaner and prolongs the life of the engine oil.
  • open engine

  • Anti-wear agents – form a protective layer on engine components to prevent the friction caused by metal on metal components. They also act as an anti-oxidant. Like detergents anti-wear agents get used up.
  • Corrosion Inhibitors – engine parts are prone to corrosion (rust). Corrosion inhibitors create a film over parts of the engine to protect them from corrosion.
  • Friction Modifiers – help to lower friction when the engine is operating under high temperatures or heavy loads. They also help to conserve fuel, however they do lose effectiveness over the life of the engine oil.
  • Graphite or molybdenum are often utilised for this purpose.

  • Pour-Point Depressants – Prevent the hardening of wax particles in motor oil under cold conditions. They enable engine oil to flow freely at low temperatures without needing the engine to work harder to pump the oil.
  • Anti-foam additives – prevent air from getting compressed in the engine oil and causing the formation of foam. When motor oil foams it is unable to successfully coat all parts of the engine.
  • What’s more in engines which have a variable camshaft timing the motor oil must also act as hydraulic fluid, which aerated oil is unable to do.

  • Zinc diakyl-dithiophosphate (ZDDP) – ZDDP was once commonly used as an additive within engine oil and acted as an anti-wear agent. However, ZDDP can negatively affect catalytic converters (which are now present in most cars) so its use has slowly waned.
  • However some older vehicles that have a flat-tappet camshaft or have an altered engine with higher spring pressures still need an engine oil that includes ZDDP.

    Different engines need different oils

    Most engine oils on the market today come with pre-added engine oil additives. However different engine oils on the market have different levels of additives, so it is important to check in your owner’s manual which engine oil is recommended for your vehicle.

    Engine oil has a shelf life and over its life, the additives within engine oil degrade. Once engine oil additives begin to degrade the engine oil starts to become less and less effective.

    Regular oil changes are therefore really important to ensure your oil is always working at its best. If you book an interim service at Stoneacre we provide an oil check as standard to help look after your engine.

    Please note we record all our calls to ensure that we give you the service you deserve.