Since automatic transmission fluids contain detergents and help fight sludge, some enthusiasts add a small amount to their motor oil prior to changing oil as a way to clean accumulated deposits and dissolve sludge. ATF, however, is not formulated for use in automotive engines and shouldn’t be used to replace a traditional engine-flush or cleaning product for several reasons.
First, the detergency, or cleaning power, of ATF is much less than motor oil. ATF isn’t exposed to the same levels of combustion byproducts or contaminants as motor oil, so they are designed with much less detergency. In most cases, adding ATF to motor oil reduces cleaning power. Instead, ATFs have elevated levels of friction modifiers and other additives that help protect gears and clutches.
Second, ATF can disrupt the engine oil formulation and reduce its effectiveness. A good motor oil is a fine balance of base oils and additives designed to work together to fight wear, reduce friction, prevent deposits and slowly dissolve accumulated sludge in older engines. 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 oil chemistry and possibly negatively affects wear protection, oil life or more.
Finally, ATF can alter the viscosity of the oil, reducing wear protection.
If you want more immediate sludge removal and cleaning power, use AMSOIL Engine & Transmission Flush. Although not a requirement prior to switching to AMSOIL synthetic motor oil, Engine & Transmission Flush is recommended to prepare engines with accumulated deposits or an unknown maintenance history for AMSOIL synthetic motor oil.