yea guitars are runed up in 4ths, and power chords are 5ths, for solo/licks i like 3rds a lot, like a lick from Silent Night, Bodom Night like this.

E - 3-0-0-0-0-0-5-0-0-0-0-0-7-0-0-0-0-0

he uses a basic scale format goin up the fret board 3-5-7 droning it with an open E. Th harmony then goess like this.....

B - 5-0-0-0-0-0-7-0-0-0-0-0-8-0-0-0-0-0

Which is cool cause hes doing a harmony in 3rds with the actual fret'd notes but then harmonizes the drone in 4ths. gives it a pretty cool sound.

Also listen to Gary Hoey - Hocus Pocus that lick you does down the fretboard is in 3rds, its not 2 guitars harmonizing but its still using intervals.

-12----12---10---10----8---8----7--7----5--5---3---3--------------
----13---------12--------10-------8-------6-------5----0-4-5------

__________________