Early models used a dipstick with a single o-ring seal.  These are prone to leaking.  Volvo has superseded the early style in to the later style but both the dipstick and the tube will need to be replaced if your Volvo is equipped with the earlier style.  

The later style dipstick is thicker at the top and will not fit in the earlier dipstick tube and the early style dipstick is loose in the later style tube.