DeoxIt D5.
Recently started using it, and I'm 100% sold.
Ingredients: a proprietary deoxidization component, no odor mineral spirits.
What it does:
Re-vitalizes, to a reasonable degree, the bright qualities of a newer string.
Put a new set of strings on a 4 string JB. The E string sounded quite dead, while A was almost annoyingly bright. One treatment, and things are much more balanced now. E became slightly clearer while the brash A mellowed just enough for the set to remain on the instrument.
On my 5'er: debated string replacement a few days ahead of weekend gig. Deox'd them, and the day before the show, I decided a new set was being wasteful, as this stuff honestly gave it that one more gig amount of clarity.
Mentioned the mineral spirits, as this is quite likely the "lubricant" that put the "fast" in Fastfret, and this stuff seems to do that as well. It's the deoxidization component that has me reaching for it over the given choices, as it really seems to do great things for string cleaning/brightening.
Final evolution: In modding my JB, used it on the pickguard fasteners, bridge assembly, and even treated the frets. Dang thing glows like brand new.
Couldn't be happier.
A couple ways to use it: DeoxIt D5 Wipes, DeoxIt "AxeWipes", or simply damp a clean cloth and apply. Buff with microfiber and let the deoxidization begin. No ill effects perceived regarding interaction with different fretboards and instrument finishes.
Finally, for me, something that works. Do a Google search and you'll see several archived TB discussions about this stuff and some interesting forms of it that have come and gone.
I know, reads like an infomercial, but I'm not affiliated, just plain happy, 'cause I'm a bright string lover, and at closed to $50 a set, glad I can extend them just a pinch further.
To coin a phrase:
"I put that sh@@ on everything..."