Any wax and grease remover should do it. I would soak the area really good with it a couple times though.I've really enjoyed the painting info in this thread. So much so that I ran out to the garage and started working on my semi primed 86 GT. Imagine my shock when i found what looks like an oil stain on the hood's etching primer (sherwin williams). What is the best method for removing the oil? Prepsol, sand prepsol, reprime? Probably an oily rag to blame.
I know primer is porous and will allow the oil to soak in it, but the wax and grease remover will cause the oil to float to the top. Be sure to wipe the wax and grease remover off each time with a clean rag or paper towels. Keep flipping the rag over and wipe until the surface is dry.
I use the Sherwin Williams line here at work, it's a very good product. They have a new clear coat that I love spraying. If you want to look into it, it's number is 1100755. Probably pretty pricey though.