It could also be from the exhausts where the cars have been running in the showroom. It's unlikely to be rubber, unless they've been wheelspinning the new motors in the showroom (as an ex car salesman, I wouldn't rule this out..).
More likely to be, as you mentioned, tyre blackener which valeters have used. I'm not sure whats in these products though.
As for being oil, it's not impossible that one of the fitters let a bit dribble onto manifold etc while they were doing the PDI, not necessarily an oil leak.