Always run at least one tear off lens, even on the hottest driest day with only one other bike on the track, having an extra layer of protection over your goggle lens will improve its lifespan. On muddy days always be prepared with plenty of tear offs to avoid being filled in and throwing your goggles off the side of the track mid race. Another helpful item riding in super muddy conditions are the Roll-Off system, most major motocross goggle brands manufacture these now which give you clearer vision for longer, many also come with a lens rain visor to prevent water seeping down in between your lenses.