Can I Use Transmission Fluid in Oil to Clean My Engine?

ATF is not made to clean sludge from engines. Instead, it's best to use a dedicated engine flush.

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AMSOIL Engine & Transmission Flush
Brad Nelson
by Brad Nelson
May 5, 2022

Over time, engines can accumulate deposits that reduce power and performance. Since automatic transmission fluid contains detergents to clean sludge, some DIYers add a small amount of transmission fluid in their oil to clean the engine prior to oil changes.

ATF, however, is not formulated for use in automotive engines. Instead, a product that’s designed to flush engines is a far-superior approach.

How Sludge Forms

Stop-and-go driving, prolonged idling, trips too short for the engine to reach full operating temperature, towing, the ingestion of airborne dirt, fuel dilution, water condensation and oxidized oil all can promote sludge build-up in motor oil and transmission fluid.

Sludge clogs narrow oil passages, restricting oil flow to vital parts, especially the upper valve train area, causing wear.

Transmission Fluid In Oil Reduces Cleaning Power

In most cases, adding automatic transmission fluid to motor oil reduces cleaning power. That’s because the detergency of ATF is less than motor oil. Why is that?

ATF is exposed to lower levels of combustion byproducts and contaminants than motor oil, so it is formulated with reduced detergency. Instead, ATFs have elevated levels of friction modifiers and other additives that help protect gears and clutches.

Transmission Fluid In Oil Reduces Wear Protection

Second, adding ATF alters the motor oil formulation and reduces its effectiveness.

A good motor oil is a fine balance of base oils and additives designed to fight wear, reduce friction, prevent deposits and slowly dissolve accumulated sludge. The cleaning power of motor oil is designed to work gradually over subsequent oil changes and not necessarily all at once. Adding a foreign substance to the oil disrupts the formulated chemistry, negatively impacting wear protection, detergency and more.

Changes Motor Oil Viscosity

Finally, ATF can alter the viscosity of the oil, reducing wear protection.

Viscosity is the oil’s most important property, influencing wear protection and fuel efficiency. Increasing or decreasing viscosity by adding a little transmission fluid in oil to clean sludge can lead to increased engine wear.

Use An Engine Flush Instead

If you want effective sludge removal and cleaning power, we recommend using AMSOIL Engine & Transmission Flush. It helps to restore fuel economy, increase operating efficiency and reduce emissions in gasoline and diesel engines, and automatic transmissions. Its potent, detergent-based formula cleans sludge and deposit build-up.

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AMSOIL Engine & Transmission Flush

  • Prepares engines and transmissions for new oil
  • Removes deposits and sludge for improved efficiency, fuel economy and lower emissions
  • Prolongs vehicle life
  • Detergent-based formula is environmentally friendly
  • Compatible with seals and gaskets
  • Works in one treatment
  • Easily disposed of with waste oil

Benefits For Gasoline And Diesel Engines

  • Helps loosen sticky valves and rings, minimizing blow-by and reducing emissions
  • Helps quiet lifter noise
  • Promotes lower operating temperatures through sludge removal

Benefits For Automatic Transmissions

  • Cleans deposits in oil cooler and ports
  • Helps unclog fluid passages
  • Cleans deposits and varnish from clutch plates, helping improve efficiency
  • Promotes smoother operation and transmission life through reduced shift delay
by Brad Nelson

Brad Nelson is a staff writer for AMSOIL. Outside of work he enjoys family adventures, wilderness exploration and riding/wrenching on vintage metric motorcycles.

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AMSOIL Engine & Transmission Flush

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