Question

Why is it difficult write on an oily paper

Answer 1

hi.....Because both the graphite in pencil leads, and the ink that is in most pens, are also somewhat oily. Therefore, when trying to leave a trail on the surface of a piece of paper, it helps if the paper is absorbent, or at least porous, as most writing papers are. Other surfaces, though, such as oily butcher paper, certain parchments, and the like, reject that ink , or give the graphite nothing to adhere to, in both cases, it makes it very difficult to leave your mark behind.
For those few inks or paints that are water based, it makes the situation even more difficult, as now the water-based product floats on top of the oil, preventing it from reaching the paper and leaving any marks.
This is why sometimes, no matter which medium you are using, bearing down hard with your writing instrument may help leave a mark: you are making the ink physically bond with more of the paper, through pressure, enabling more of the ink to be left behind.
I am sure someone can better explain the chemical processes going on here, but this is a simplistic answer that should suffice.

hope this helps