For having used "sulfonitric acid" (sulfuric and nitric acids mixture) for at least 2 decades, PTFE is definitely stable to such a mixture.
Safety on the use of "sulfonitric acid"
I know it is off-topic but I think that safety should be learned as light touches, not as a-difficult-to-swallow course. (Please tell me if it is not in accordance with the site's rules).
You can safely use "sulfonitric acid" (i.e. usually a mixture of one volume of conc. sulfuric acid poured slowly into one volume of conc. nitric acid) to clean glassware and PTFE-ware if you follow some common sense rules. Explosions (or at least too vivid reactions) reported were the direct consequences of a poor understanding of what "sulfonitric acid" does.
- Handle it under a fume hood (I told you it was common sense!!)
NEVER pour sulfonitric acid on organic material as the latter will be oxidized in an explosive way! Every piece of glassware should first:
- Be cleaned as much as possible with any solvent or manually removing solid substance which is stuck where it should not be,
- Be thoroughly rinced with a solvent miscible with water (acetone, methanol, ethanol...),
- Be thoroughly rinced with tap water, in order to eliminate most of any organic solvent,
- Only then can you pour the sulfonitric acid onto your glassware and let it do its magick.
Do not think that a plastic which can seemingly resist any solvent will actually resist exposure to sulfonitric acid! PTFE will resist but not common plastic materials.
If you follow these rules, there is no reason why you should experience any dangerous reaction in your lifetime. Be especially careful with sintered funnels.