According to the book, a Longevity Ritual prepared on behalf of someone with a Supernatural Ability functions with strength identical to that it would provide a magus (Modifer = Lab Total / 5), while a Longevity Ritual prepared for an un-Gifted person WITHOUT a Supernatural Ability functions at half-strength (Modifer = Lab Total / 10).
There is a dispute amongst my players as to whether someone who possesses a Supernatural virtue that doesn't have an attendant Ability (for example: Unaging) is treated as a Magical individual or a Mundane individual for the purpose of determining the strength of a Longevity Ritual.
The wording in the text is pretty unambiguous that it only applies to people with magical Abilities, but I have to confess, this does seem just a little strange to me, so I figured I'd consult the board and get their opinion on the issue.
Edit: Forgot that Unaging is apparently General, not Supernatural (?!). Fixed my example.
Edit 2: No, the index of Virtues and Flaws just misplaced Unaging. Changed my example back.
By a strict reading of the RAW, only a supernatural Ability (e.g. Dowsing) will do, and any supernatural Ability, even Faerie, Infernal, or Divine, will do.
The other way I've seen it played is the following: to gain the greater bonus, a magical virtue orflaw* that is "intrinsic" to the character is both necessary and sufficient. I still have to find a good definition for "intrinsic" Virtues and Flaws, but as a rough guideline they are those that do not depend on an external factor: supernatural abilities count, as does unaging, magical air, werewolf etc. - but not maledictions, magical companions, magical items etc. Thus, Visions grants the greater bonus, but neither Ghostly Warder nor Sense Holiness/Unholiness do - the former because it is not "intrinsic" to the character, the latter because it is either a Divine or an Infernal virtue.
I'd bet you dinner that the text was just sloppy. I can't think of any reason to discriminate between characters with Supernatural Abilities versus those with Supernatural Virtues with no corresponding Ability. Is someone with Dowsing more "magical" than someone with Greater Immunity? I don't think so.
Only if you have a Mystagogue available who wants to initiate you, and you can/want to do the initiation. The RAW don't make any comment on how rare or common this happy circumstance might be; that's very saga dependent.