I can't see any need to treat leaders or tippet material.
Fly line that is still fairly fresh can usually just be cleaned with some mild soap and buffed dry with an old T-shirt until it is nice and slick again.
Line that has lived a hard life under the sun and/or under your feet can be kept usable long enough to raise the 40-70.00 needed to buy a new one by cleaning, drying, and rubbing something slick and water resistant on it.
WD-40 and Armour-all don't work at all. Neither do those little pads of "fly line cleaner" that come with some lines. But I have used Silicote reel lube and Vaseline with good results on cracked-up aged fly line even though I guarantee somebody is fixing to come along shortly and tell you that those two things (as well as any other easily obtainable lubricant) will absolitely cause your line to disintegrate before your very eyes, because everybody is taught to believe that a fly line is some mystical, magical thing made of unobtainium gathered from a galaxy far-far away and highly vulnerable to most everything on earth.