When is it appropriate to use ATF in diesel fuel? My Dad used to use ATF to add lubrication to the diesel in his farm tractors.
The use of ATF in diesel fuel is debatable. Certainly it will provide lubrication for the fuel pump and injectors but the non-combustible antiwear additives will add to the deposits in the combustion chamber. Transmission fluids are made to lubricate gears and bearings. The typical additives for antiwear are Boron, Calcium and Phosphorous. These additives are not designed to burn in the combustion so they will deposit on metal surfaces. Maybe, there is a threshold of concentration as some have mentioned where the deposits are not a problem but I don't know what that concentration is. There are excellent fuel additives that will out perform trans fluid in the combustion chamber and provide extra lubricity for the fuel pumps and injectors.
In the final analysis, there are additives for the fuel that are made for the combustion chamber. Run good quality diesel and you will eliminate the need to run a lot of additives but when you need one, run a fuel additive.