Motorcycle riders have been lubing brake and clutch
cables on their machines for decades. The "smart" ones use a lubricant
designed for the job and a small tool to force the lubricant down the
cable.
So, I agree, DO NOT use WD-40. It is more of a
cleaner than a lubricant.
When I rode (before the advent of hydraulic clutch
and brake cabling) I used one of these devices and can tell you that they
work and work well...
Not bad for a 17-buck deal (shipping
INCLUDED)...