The Mazda 6, CX5, CX7 that using engine code of LF, L3 and L5 have a very special design of the oil cooler, there are 2 pieces of parts may related to gasket side, one is the metal piece of gasket and one is the rubber o-ring, so you may find where this gasket and o-ring located at the oil cooler from the photo.
As for the rubber o-ring, they have one older version of plastic type of gasket, which is similar shape with the o-ring but their material is plastic, it seems after some while Mazda may think the rubber type of material is more suitable as the sealing method on that area, hence they had change the old plastic type of gasket to the current rubber o-ring type.
