Update: I called dupont's authorized service center in Rhode Island.
They said they don't service the X-tend model (discontinued parts), and offered me to exchange it for a new lighter for $100.
(she also spouted some stuff off about small parts and too hard to service, they just change the whole tank assembly,
when they could still get the parts in)
(history note: some years back Dupont discontinued the made in France xtend models and introduced a very similar looking made in China Maxijet model)
Now, I'm rather attached to this lighter, not a new one.
Nothing to lose, out comes the screwdriver...
"small internal parts" uhhhh huhhhhhhh
Turns out the lighter is pretty simple, and not at all small.
unscrewed the leaking valve part, but couldn't get the little valve tube any further apart to directly service the o-ring.
Plan B - squirted some WD40 on the the fill valve nozzle (again, nothing to lose here) and worked it in good. give it time to air out and dry.
then with the nozzle still off the lighter and gripped in a pair of pliers, ran some butane through it to blow it out.
Re-assembled, filled, no more leaking lighter!!
Of course, time will tell wether any of the wd40 gets into the flame nozzle and clogs it,
I'm hoping not.
But had nothing to lose. So far, so good!