Most people that use automatic transmission fluid (ATF) in a diesel engine use it as a fuel additive rather than as a standalone fuel. The intended purpose of the addition is to use the ATF's lubricating and cleaning properties to clean out the engine and improve overall performance. Tests have indicated, however, that adding ATF does not provide any discernible benefit.
Many diesel engines, particularly older ones with looser fuel tolerances, are capable of burning and running on pure ATF in place of diesel fuel, and a small number of people collect and filter used ATF for this purpose.
Newer, computer-controlled engines may have difficulty with any fuel that has significantly different physical properties from standard diesel fuel. ATF does not burn as readily as diesel, and it is much more viscous. While the engine may run on ATF for a time, doing so over the long term may cause significant damage to the fuel system. And because ATF is not a government-approved fuel, it could lead to legal consequences for the user.
