Its something i usually use at the construction phase and works quite well. I wipe a small amount into the fretboard, leave for 5 minutes and buff off the excess. I can honestly say it gives fretbaords a fanstastic look and feel

It is a polymerized linseed oil, which means it will set in the wood more than a raw oil ever will - thats why its better for dry fretboards . . . i wouldnt quite call it varnish though. It is an accepted finish for guitars that works well on fretboards as long as it is used spareingly once the guitar is in use