Question

You can't write on a sheet of paper with an oily surface

Answer 1

Because both graphites carry the pencil, and the ink found in most pens, is also quite greasy. Therefore, when trying to leave a trace on the surface of a piece of paper, it helps if the paper is absorbed, or at least porous, as most papers write. Other surfaces, though, such as oily flesh paper, certain solid paper, etc., reject the ink, or give any graphite to be observed, in both cases, it makes it very difficult to leave your mark behind.

For some water-based ink or paint, it makes the situation more difficult, as now water-based products float on oil, preventing it from reaching the paper and leaving any mark. That's the reason sometimes hard-paying with your writing instrument can help leave the mark making a physical bond in bonding with more paper, through pressure, allowing more ink to be left behind. 

Hope it helps you :)