While both of these are in Oils, "DECEMBER DUSK" was rendered using brushes and "WESTERN VISTA" was created solely with painting knives. The difference in the surface textures is clearly evident in these photos.
Sometimes, I prefer a smoother finish and on other ocassions I desire a more heavily textured interpretation. It's the best of both worlds, really, and it's nice to have the freedom to represent something in the way that I see it. Too often, an artist can settle into a comfortable "rut" , which translates into predictable results. Try to shake things up by doing something different once in a while, whether it's using an unfamiliar brush or a different medium altogether.
As for using the knives, I cannot say enough about my fondness for Lukas Painting Butter ( available exclusively through Jerry's Artarama and Art Supply Warehouse ) It's an alkyd-based impasto medium which is fantastic for building up heavy paint layers, such as those shown here. Give it a try, you'll like it!