Here and here and of course Grognardia with 83 comments.
It was DM'ing kids D&D, watching my young players all buy silver daggers, and then silver swords (100gp) in case they faced a wight. After the first session no wight but they all had just over 100 hard won XP. And I thought, losing that XP would be a tough loss, and then I had for me a eureka moment. An elegant solution to all the debate. Well for my campaigns anyway.
Undead will drain XP not Levels and each hit will will drain the XP you might have gained for killing the undead creature. So a wight in B/X is worth 50 XP. Therefore each hit by a wight drains 50 XP.
Still something players will seek to avoid, no one likes to lose XP, but it is not the total downer that level drain is. Zero level NPCs still die with one hit. Higher levels can now contemplate fighting undead toe to toe. But who wants to go into melee range? Enough hits and one might lose a level but in play, most often it will be easy to wipe XP off the characters XP total, without any fiddly ability score loss, or time based effect. That's it, the XP are gone, drained away by all that negative energy.
You want a pirate ship crewed by wights and captained by a vampire. That's what I am now planning. Previously certain death unless a powerful cleric is in the party. Now... armies of XP draining undead are possible.
Previously the name level fighter would run faster away from a wight than he would a huge dragon. Level drain was that fear invoking. Now wights are for low level characters, en masse for higher. And that sits very comfortably with me.
Wight = 50 XP drain / hit
Wraith = 175 XP drain / hit
Spectre = 725 XP drain / hit
Vampire = 7 HD (1250 XP drain / hit), 8 HD (1750 XP drain / hit), 9 HD (2300 XP drain / hit)
(or if you want to be double drain for Spectre and Vampire like in the rules = double the XP drain, a couple of hits from a vampire will make most players cross but not quitting!)
You could easily apply for AD&D as long as you know the hit points. Page 85 DMG.
AD&D undead generally hit harder than B/X undead.