Both can be drilled to handle oil, but the original No Mercy handles surface changes better, so henceforth is a better selection for handling heavy oil (if that is what is meant by an "oiler").  Both balls come in at 4000 abralon, so if you are going to go with either one for heavy oil, I would suggest taking the cover down to around 1000 abralon.  Good luck!
--------------------
MainePBA
"Power tends to corrupt; absolute power corrupts absolutely." Lord Acton