They come Pre Oiled and the "Recharge Kit" (includes cleaner and oil) is readily available every where and unless you live in a extremely dusty area they are good for 20,000 km at least, rain dose not affect them some have put covers on but mine are fully exposed and stick out further and I have been in torrential downpour with them the little one come with oil only because they are packed unoiled and don't need to be the breather will soon oil itself and the temp senor is more to protect it I oiled mine then realised they really didn't need to be done
Oh and Repco regularly has the "Recharge Kit" on sale in the sale catalogue