Understanding oil oxidation – What causes engine oil to thicken over time?

Engine oil does far more than simply lubricate moving parts. It helps protect against wear, keeps components clean, and supports overall engine efficiency. However, like any hardworking fluid, engine oil changes over time.

One of the most important processes affecting oil performance is oxidation, and if left unchecked, it can lead to oil thickening and reduced engine protection.

As oil molecules react with oxygen, they form acids that may corrode engine components, along with sludge and varnish that create sticky deposits inside the engine.

Oxidation also produces reactive compounds, such as peroxides and aldehydes, which further degrade the oil and weaken its protective properties. Over time, these oxidation by-products accumulate and begin to alter the oil's structure – and this is where thickening starts.

As oxidation progresses, heavier compounds and polymerised molecules build up within the oil. These increase its resistance to flow, making it more viscous. Thicker oil moves more slowly through the engine and may struggle to reach tight clearances fast enough.

Reduced flow can compromise lubrication, increase engine wear, and even contribute to higher fuel consumption. In short, oil that has thickened due to oxidation can no longer perform at its best.

Several operating conditions can dramatically speed up oxidation and oil thickening, including:

• High operating temperatures – oxidation rates increase significantly with heat; even a 10-degree rise can dramatically accelerate the process.
• Extended oil drain intervals – the longer oil remains in service, the more time oxidation has to develop.
• Frequent stop-start driving – repeated heating and cooling cycles place additional stress on the oil.
• Contaminants and metal particles – these can act as catalysts, encouraging oxidation and sludge formation.
• Lower-quality oils – formulations lacking robust antioxidant additives can degrade more quickly.

Using a high-quality engine oil formulated with strong antioxidant additives helps resist oxidation and maintain viscosity stability over time. In demanding driving conditions, shortening oil change intervals and avoiding excessive idling or repeated short journeys can also help slow the vehicle ageing process.

In any case, always consult your vehicle manufacturer's recommendations when selecting the correct oil grade and determining the appropriate drain interval. 

Find out more about Texaco Lubricants’ range of engine oils.

About Rudi Sanders

Based in Ghent, Belgium, Rudi Sanders has worked for Texaco Lubricants for over 30 years. Across his career, he has held an array of positions, including being the laboratory manager of the chemical and physical test laboratory, country manager and manager of sales and marketing for Benelux and France.

Rudi is also currently chairman of the Industry Liaison Committee for ATIEL (Technical Association of the European Lubricants Industry) and co-chair of the ATIEL liaison committee for light-duty vehicles.