@Kubalson
My two cents.
An oil slick itself is not yet a conclusive evidence of the crash site. I remember very well in 2009, during the early stage of the search for AF447, a long oil slick was also spotted by a Brazilian Air Force rescue aircraft. Eventually it turned out the slick was probably the result of ship leakage.
The actual debris area was found many miles away from there.
So it may be an evidence of the crash or may be not.