I do not think it is oil. My 1995 Avalon have been doing this for 3 straight years and only when the outside temperature is hot. You would think after 3 years what is there would have been gone, but it isn't. I use moist towlettes to get rid of the eyesore, albeit, this is temporary. It eventually comes back, especially during the summer. The substance is too sticky to be oil. It looks like some kind of adhesive.