Good question. Is oleum used for liquid fat, no matter the source, or is it confined to plant-based oils? pinguis has definite animalistic qualities. You'd think it was oleum nucis indicæ [1], not pinguis nucis indicæ, except when used in a metaphorical manner.
I provided the link to google books to the Liber fundamentorum pharmacologia only to show that oleum serpentis was actually an ancient remedy, of course the Latin of a book translated from medieval Persian might be not exactly Cicero, still it should be much better than any translation I can do.
But most probably oleum was a synonym of olive or however vegetable oil in ancient Rome, and it is entirely possible that the actual Persian medicine was the extract of some plant and only called serpentis.
On the other hand, besides the name, we don't actually know if snake oil is actually made of snake oil or snake fat or something else.
Paging Reginald Foster, Father Reginald please!
[1] nux indica is the coconut