parts washer fluid questions

Call your local petroleum wholesaler and ask for "Stoddard Solvent" aka odorless mineral spirits, aka Mil Spec P-D-680 Type 1, aka Varsol, aka VM&P thinner. They're all about the same stuff. Naphtha is a catch all term for several petroleum distillate hydrocarbons.

Last time I refilled my cheapo Harbor Fright tank I think it cost me about $45 for 10 gallons (my cans) of mineral spirits. The stuff will evaporate if you leave the lid open, but is less stinky than diesel. In terms of volatility, it's similar to kerosene/Jet A so it should be treated with some respect, but not a serious fire hazard compared to Gumout or lacquer thinner.

I've never added ATF (gloves are your friend) as the fluid tends to get oily with time. In fact once it gets some oil in it it no longer evaporates clean and loses its efficiency. I try to drain and degrease parts before I throw them in the tank to prolong the life of the solvent. Also, the old oily solvent is hazardous waste much like used oil and should be disposed of accordingly (any place that sells it is probably required to accept the waste).