Was John ever put in boiling pot of oil and came out still preaching?
Tertullian in A.D. 200 wrote, "at Rome, the Apostle John, having been immersed in hot oil, suffered no harm at all from it" (The Prescription against Heretics, chapter 36). The claim is that because John wasn't killed that it lead to his banishment to Patmos. Thus, it is an old story, but our record of it is 100 years after the claimed event and it wasn't recorded by inspiration. Jerome in A.D. 397 refers to Tertullian's account, but makes the mistake of adding to it, claiming that it was Nero who ordered John's execution (adv. Jovin, I.14). This is a sign of a fable when embellishments are added -- especially impossible ones (Nero was dead when this event was supposed to have taken place) that come from people not living during that time. It was Jerome who also said that John came back out preaching.
While there might be some truth behind this fable, it has too many markings of embellishment -- in this case trying to explain why John was on Patmos (Revelation 1:9). For that reason I would treat it as a fable unless better evidence surfaces.