There's a variety of painting materials you can use to add impasto texture to your oil paintings. Generally, texture looks best in the areas of the painting that are brightly illuminated and light in value.
You can just add thicker oil paint, as I did on this detail of Waterfall City from Dinotopia: Journey to Chandara. In this case the paint is applied with a palette knife.
If the paint comes out too runny from the tube, you can squeeze it out on blotter paper or paper towel to draw out the oil. Thick paint takes weeks to dry, so a drop of cobalt drier mixed into the white will make it dry in a day or two. Just use a drop, because too much cobalt drier can affect the color of the mixtures.
In some previous posts noted below, I covered "prextexturing," where you add the texture first before final painting and then paint relatively thinly over that base texture, and glaze into the pits.
But what are your choices for this pretexturing? Here's a test with an assortment of materials. At the top is acrylic matte medium, modeling paste, and gesso, mixed together in various combinations. Acrylic paint is OK to use in the priming stage, before beginning the oil layers, but never add it over the oil, or there might be adhesion problems.
In the second row of test swatches I experimented with Wingel and Oleopasto, two Winsor and Newton products that are designed for quick drying impastos. In both cases I let the textures dry and glazed over the top. The white streaks on the test swatches are where I rubbed off the glazing layer, leaving some of it in the small pits.
Previous GurneyJourney posts on the topic of pretexturing:
Rembrandt Effect, link