I've never heard it was boiled in oil!
Could you give a citation or two?
Anyone?
No, it was never boiled in oil.
Search for the bolded heading "Carbon Dating the Shroud: charred areas" and then read paragraph 2 below that.
(To be fair, however, Raymond Rogers on this site finishes by saying, "The image is not a scorch. The cloth was not heated, not even boiled in oil.")
Your mileage may vary: it is important to recall that the Shroud was not kept in a strict forensic chain of custody since it was first discovered.
Cheers!