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.