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It is not a bad practice as it protects against rust, but shouldn't be needed if you keep as much moisture out of your air system as possible(draining the air tanks often or installing an air drier).
The new style airpacks don't have that port to ad oil to.
Its been too long since I've been into the old style.
Search for threads started by Rustystud, I'm pretty sure he has a step by step rebuild.
Edit, it was deuceswild:
http://www.steelsoldiers.com/showthread.php?82753-MY-Airpack-Rebuild-with-Photos!&highlight=air+pack+rebuild
Pull it apart before getting the rebuild kit, many are too rusted to fix.
If thats the case, I'd get the short style. Less parts to rebuild when the time comes.