Could you heat one up and see what runs out. If it is oil, then it should run out.
I'd be surprised though, I wouldn't have thought it would take much oil, and she'd be smoking like a steam train.
Would it be worth running a bit of bio through a pure white cotton cloth, just to see in what way it discolours it.
Could maybe take a sample at different points along the fuel system and do the same thing, see where it is coming from.