I've had a look and Nut Tuning Lubricant coast more or less £6.
Is it worth buying it or does the graphite pencil trick do the job fine?

how long does the graphite trick last? i.e. how often would I have to do it?
And the same question about the Nut Lubricant.

One of my guitars seems particularly prone to this problem.
yes, and you just use a tiny bit at each contact point. whenever you change strings.

It's made not to damage your guitar.
all my eyes saw were NUT and LUBRICANT, and i lol'd.
Quote by candysars
Thank you Jesusaurus.