It was suggested as a treatment to unclog/dissolve junk in the transmission of our car.
And that person smokes meth for better health, too? Mineral spirits or whatever composes thinner burns off very easily. I would be more worried of a flash fire than anything else.
No, that is not a safe treatment for a transmission.
Treat a transmission? If you've disassembled the transmission and are cleaning individual parts, perhaps, although there are safer solvents to use. If you're contemplating flushing the gunk out of the whole unit, forget it. You'll be far better off just leaving the gunk where it has settled. Most car manufacturers specifically advise against transmission flushing, no matter how profitable it is for the quick lube shops. Don
