Check out this video on making the best wood fill I know of. Gather fine sawdust from your shop or recent woodworking project and store it in a clean container until ready to mix up a batch of wood fill. Mix with expoxy (either 5 minute or West System expoxy) on the wood surface that needs to be filled and adjust the consistency as needed by adding more sawdust to thicken, or add more epoxy to thin. Let it dry for 30 minutes or so before sanding. This mixture can also be shaped much better than the wood fill you buy.
TIP : To fix an imperfection in wood like walnut, cherry or mahogany, mix the sawdust with some charcoal shavings from artist charcoal that you rub off using sandpaper. Add the charcoal to the sawdust to create a natural dark finish. (Check out how to do this at the 2 minute 53 sec. point of the video). I use the natural sawdust mixture when working with maple.
Good luck and hope this tip helps.