How Engine Sludge Forms. And How To Prevent It.

Engine sludge is a black gelatinous substance that wreaks havoc in engines. Fighting sludge in oil means using a good...

November 9, 2021

Engine sludge. It’s a black gelatinous substance that wreaks havoc in engines. And long before the engine’s demise, engine sludge can foul engine sensors and interfere with performance. Some mechanics call it the “black death.” So, how does sludge in oil develop? And, how do you prevent it? Here’s what we’ll cover:

  • How engine sludge forms
  • The effects of sludge in oil
  • Synthetic oil helps prevent engine sludge
  • High-quality additives fight engine sludge
  • Severe service invites engine sludge
  • How engine sludge forms

    Engine sludge is the result of a series of chemical reactions.

    The lubricant degrades as it is exposed to oxygen and elevated temperatures. The higher the temperature, the more rapid the rate of degradation.

    In fact, every 18°F (10°C) increase in temperature doubles the rate of oxidation.

    The byproducts of this reaction form highly reactive compounds that further degrade the lubricant. Their byproducts react with other contaminants, forming organic acids and high-molecular-weight polymeric products. These products further react, forming the insoluble product known more commonly as sludge.

    What begins as a thin film of lacquer or varnish builds up on hot or cold metal surfaces and bakes into an expensive mess.

    The effects of sludge in oil

    Sludge can block the oil passages and oil-pump pick-up screen, resulting in oil starvation. Often, the negative effects are cumulative rather than sudden.

    Many engines with variable valve timing (VVT) use oil-pressure-operated mechanical devices to change valve timing, duration and lift. Sludge in oil can plug the solenoid screen or oil passages and affect the operation of VVT mechanisms, eventually leading to a costly repair bill. Sludge reduces efficiency and increases time and money spent on maintenance.

    Engine sludge can wreck variable valve timing components.
    Engine sludge on a variable-valve-timing (VVT) solenoid.

    Synthetic oil helps prevent engine sludge

    Fortunately, sludge and varnish deposits are something oil manufacturers can control. Using thermally stable synthetic base oils reduces the rate of degradation (oxidation).

    Anti-oxidant additives help reduce the rate of degradation as well. One of the most widely used is zinc dithiophosphate. Not only is it an excellent oxidation inhibitor, it is an outstanding anti-wear additive as well.

    High-quality additives fight sludge

    We can further address many of the issues occurring after the initial oxidation stage.

    Additives, such as detergents and dispersants, are commonly part of the motor oil formulation. They help promote the suspension of contaminants within the oil and keep them from agglomerating.

    Detergents, which are also alkaline in nature, assist in neutralizing acids generated in the sludge-building process. Anti-oxidant, dispersant and detergent additives are consumed during use.

    To achieve maximum life expectancy, use an oil with high concentrations of anti-oxidant, dispersant and detergent additives.

    AMSOIL Signature Series Synthetic Motor Oil, for example, has 50 percent more detergents* to help keep oil passages clean and promote oil circulation. It provides 90 percent better protection against sludge.**

    Signature Series Synthetic Motor Oil was subjected to the Sequence VG test to measure its ability to prevent sludge. Signature Series produced an oil pick-up tube screen virtually free from sludge. Its unique combination of detergents and high-quality base oils control oxidation and sludge to keep engines clean and efficient.

    Oil pickup screen free of engine sludge.
    AMSOIL Signature Series virtually prevented engine sludge on this oil pick-up screen.
    AMSOIL Signature Series

    Severe service invites engine sludge

    Equipment operating conditions also influence the likelihood of sludge in oil or varnish issues.

    Stop-and-go driving, frequent/long-term idling and operation in excessively hot or cold weather can increase the likelihood of sludge and varnish, especially if using more volatile conventional oils. If sludge has already formed, you can use an engine flush to clean sludge from your engine.

    Most auto manufacturers note in their owner’s manuals that operation under any of the above conditions is considered severe service and requires more frequent oil changes.

    From a mechanical standpoint, things like adding too much oil to the oil sump, antifreeze contamination, excessive soot loading, excessive oil foaming, poor engine-combustion efficiency, excessive blow-by and emission-control-system issues can all lead to the formation of sludge and varnish.

    By practicing good maintenance and using properly formulated, premium synthetic lubricants, your vehicle won’t succumb to the “black death.”

    *vs. AMSOIL OE Motor Oil
    **Based on independent testing of AMSOIL Signature Series 5W-30 in the ASTM D6593 engine test for oil screen plugging as required by the API SN PLUS specification.

    Updated. Originally published March 8, 2017