Located in Filters Artistic Oilify, this filter makes your image appear as if it has been painted with oil paints.
Oilify has two options: Use Intensity Algorithm and Mask Size. The differences between an image created with the Use Intensity Algorithm checked and one without it are hard to detect. It appears that with the algorithm more details and better colors are preserved. Also if the image being oilified was already a JPG, it is recommended that the algorithm is used because the default method tends to make artifacts more visible.
Mask Size basically sets the size, in pixels, of the brush used to paint the image. It determines how large the splotches of "paint" will be.