It's a T6 Torx. Before you loosen it, though, check and see how freely the blade moves with the lock sprung back all the way and not contacting the blade. If there is significant difference you might want to leave the pivot alone and just oil it.
Loosening the pivot doesn't have much effect on these knives. The locking leaf puts a lot of pressure on the blade and presses it pretty tightly against the inside of the opposing handle liner. The friction will cause quite a bit of drag on the blade.
A good lube such as Militec-1, Tuff-Glide or White Lightning will help but if It's still too tight, I would recommend sending it to Buck for adjustment.