Originally Posted by JohnDThompson

Most derailleur pulleys use simple sleeve bearings, which are dead-easy to clean. Some fancier models may have sealed cartridge bearings, which shouldn't need service. And some old-school derailleurs actually use loose balls in a cup-and-cone system.

There's no compelling need to replace them unless the pulley falls apart. I have some that are 30-40 years old and still going strong.

Originally Posted by skimaxpower

They're easy enough to remove, clean, lube, and re-install. Requires a hex wrench and 15 minutes of your day.