Originally Posted by Matt93SE
Funny.. I've been using ATF in the engine and manual tranny for years to help clean it up. There's absolutely nothing wrong with it, done correctly.
Many manual transmissions specify it instead of gear oil or motor oil.
You can not correctly perform something that is incorrect to begin with.
Your point that some manual transmissions require atf is not relevant to this issue. While some manual transmissions do specify atf, those units are specifically designed for that fluid...that's significantly different than pouring ATF into an engine.
Using ATF in the engine oil to clean the crankcase is definitely not a recommended practice. The fact that people have done it and the engines have lived-on doesn't make it right. ATF has different properties than motor oil does, and it should not be used in an engine. Contrary to popular belief, ATF does not act as a detergent. When added to engine oil, ATF simply thins the oil and will allow faster draining of the oil from the engine. Then consider what the residue left in the engine is going to do to the fresh oil added to the crankcase? Diluting it certainly isn't good.
If people maintain their engines properly, there should be no need to clean them internally. However, and there's different schools of thought on this, if it is thought an engine requires cleaning internally, a properly formulated engine flush is recommended since they contain both detergent and solvents specifically designed to remove soft deposits in an engine.