When I buy a wheel's worth, my shop gives me a complimentary dose of Wheelsmith or Phil Wood spoke prep.

The idea is that it acts as a lubricant while you build the wheels and a thread locker after it's cured a bit, too keep the spokes from backing out.

Jobst Brandt claims it really was invented to facilitate machine-built wheels and is unnecessary.

Linseed oil is indeed used but I hear it needs a few days' cure time.