The idea of Naphtha, which is also used as lighter fluid, in an ultrasonic, with heat, just gives me the heebie-jeebies. It's already really volatile, and flammable, at room temp without heating and vaporizing it with the cavitation of an ultrasonic unit. I use Naphtha for last minute cleaning when reassembling mechanical watches, like if I accidentally get oil somewhere it absolutely shouldn't be, because it dries really fast (volatile).
Mineral Spirits, aka Stoddards Solvent, is similar to Naphtha, but the hydrocarbon chains are longer, so it's less volatile, while still working just as well as a solvent. Most cleaning solutions for watchmaking are based on Stoddards Solvent, because it will strip off oils and leave the surface completely clean once dried, but it won't fill the air with hydrocarbon vapors.
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