The solvent capacity on my washer is 10 gallons and I found that using 8 gallons of mineral spirits and 2 gallons of kerosene gives the best results at a reasonable price. About once a year I filter it and clean the bottom of the tub and top it off with the same 80 / 20 mix of fresh fluid.
Diesel fuel aka #2 home heating oil will eventually jell and gum from the algae that is in it. Agreeably this isn't much of a problem when you consider the other garbage we are introducing into the fluid as we clean parts but mineral spirits and true kerosene are algae free.
As far as flame safety is concerned both diesel and #2 heating oil have the same flash point as kerosene of 100 degrees F ( minimum ), and all 3 products ignition point is in the 650 degree or higher range. In simple terms those fuels have to be at 650 degrees or higher before they will self support combustion.
