Originally Posted By: 2cool
Why do you think so? Not meaning a flame, just was curious what information led to your observation.
ATF is designed to be used in an automatic transmission which is a sealed unit, that operates with relatively low amounts of boundary regime conditions, contains low amounts of AW additives, contains additives for clutch slip that will have no use in firearms, and they are typically red that will stain just about anything and stink to high heaven.
Better than nothing? Sure, but not ideal.
Motor oil has higher levels of AW and corrosion inhibitor, doesn't stink as bad, and is generally better at boundary regime lubrication which all firearms operate under.
Even motor oil will not contain EP additives that will aid with boundary lubrication. IMHO, a proper gun oil will contain EP additives to help smooth the surface of the metal and prevent galling, especially with stainless steel.