I just use Vaseline (petroleum jelly) straight out of the jar, applied with a small stainless spatula (about 10mm wide, rounded end)
but have noticed others have given different advice in previous threads, including commercial lubes.
Might not matter that much in practice.

BTW I start with .457 balls for the first couple of cylinders, then switch to .454 when the fouling makes ramming a .457 ball difficult.
In my original Tranters accuracy does not change perceptibly.