careful with the magic eraser....if its a vinyl top you may take the dye off....ME is abrasive...
it looks to some sort of grease smudge...and you may need a stronger degreaser type product..I would try a citrus cleaner...and do a test spot out of site....let the owner know that if he wants it removed you may encounter a lighter spot ..release your self of any blame....if he does not want it removed than better for you in the long run as removing the stain may cause a further spot....
did you try
ARO full strength and use a little scrub brush or tooth brush yet...get a fingernail cleaning brush(great tool to have for many uses...good for spuring foam pads also) and try that..its s little stiffer than a tooth brush....
AL