Do not skip gears on a downshift, it is not neccessary and will most certainly be a cause of damage within the box. Any vehicle with these gearboxes, must not be driven like a modern car, ie, on the requirement for a downshift, the vehicle should be slowed ( either by brakes or backing off the throttle early enough), to a suitable speed for the lower gear, before changing down, if not, you will feel a sudden snatch as the box is changed down too early. The shift done right is so smooth there is no snatch or sudden rise in engine revs.