When I bought my parts washer I went to Lowe's and bought mineral spirits. It cost ten bucks for five gallons and I got by with ten gallons in a twenty gallon washer. That sure beat spending thirty dollars for five gallons of the recommended solvent.
I've been using it like that for three years and NO problems with flammability or evaporation. Granted, it's evaporated a little and it's about time to add another five gallons (only about a gallon evaporated, but the pump is starting to draw air). It also cleans really well and I can only smell it when the lid is open on the parts washer.
On a side note, if I had put a full twenty gallons of the recommended solvent in there, I would have had more money in solvent than I had in the washer itself.