I encourage my students to not use a graphite pencil when sketching in their subject matter on the canvas or other painting supports for oil and acrylic paintings. Using a small brush with very diluted paint/solvent mixture to block in your forms is my suggestion. Remember that painting is drawing and if you can become more prolific at using a brush, your brush work and mark making in general will greatly improve.
The diluted paint that you use to "sketch" in your subject matter will dry very quickly and will not taint your paints like graphite will. If you must use a "pencil", I would suggest drawing in your subject with a neutral color watercolor pencil, which gets dissolved in your paint and will not cause any discoloration of the oil or acrylic paint.