For consideration, I'll just toss in how I've handled Level Drain since the late 80's or very early 90's.
Life Energy Levels
Every PLAYER CHARACTER (or select NPC's) has a "LEL". It is equal to their Level. When hit by a level drain, that level drain comes from that LEL number. Think of it like 'HPs for Level Drain'. When those LEL's are reduced to BELOW 0, the PC looses an actual level. His/her LEL is then re-set at his/her new level.
LEL's recover at the rate of 1 per full day of rest. A bonus LEL can be recovered that day if the PC has been at least moderately devoted to the gods or other Outer Planes creatures of significant power, like Demon Lords, Elemental Lords, Arch-Devils, unique entities like Primus or the Cat Lord, etc. The PC must perform a specific ritual that they can learn from various sources; ancient texts, 9th+ level priests/paladins, sages, etc. Once learned, the PC can use it forever more (eg: it's not a 'spell'...you can do it if you have the appropriate stuff; think of it like "eating your veggies...but for your soul"). The cost varies a little, based on material costs of the needed sacrifices/etc, but averages to be about 100gp to 300gp per day (1d6x50gp). This cost is fixed when the PC learns that particular ritual! This means they CAN learn a cheaper version...if they can find it.
If/when a PC looses all their LEL's and looses a REAL Level, that is handled as per the normal AD&D rules; a Restoration spell is needed, or Wish, etc. If a Real Level is then returned (say, via Restoration), then the characters LEL is fully restored to that regained level (e.g., Level 9 PC gets dropped to Level 8 and looses some LELS in the process, down to 3 LEL; he gets a Restoration spell cast on him; he jumps back up to his Real Level of 9th again, and his LEL is instantly reset to the full LEL 9 ).
And that's how I handle Level Drain. In a nutshell. It does two things: Keeps the soul-crushing fear of powerful undead... keeps the very special nature of Clerics and high-level Paladins as very special indeed!... and allows for the recovery of LEL to be integrated into the campaigns actual play and lore, allowing for the "powers of the multiverse" and their worship to actually get some air-time during the roleplaying session (in stead of just the typical lip service).
Paul L. Ming