I dampen a piece of tissue paper with Goo Gone, lightly rub it over the oily area, then follow up with dry tissue paper.
I had a nerve twitch when filling my Y, and squeezed the entire bottle all over the thing. Yuck.
Side thought- I wonder if that residue is sticky enough to hold on to charcoal dust? My thought is if you can get the locomotive covered with that oil "naturally", drop some dust on it and spray it with some clear flat then you've got some realistic soot weathering.