I use a motor oiler(it's designed for it)...has a long tube on it, to reach inside. You can buy them at quality hardware type stores....$3 will last 10 years.
i use hair clipper spray for mine.... ^__^ oiled it once and has been much quieter ever since (about 3 months so far)
EDIT: i might add though, if you're using vented hoods, i suggest doing the oiling at night when the lights aren't running so that you can just circulate the air in the room for an hour or two so that you don't contaminate your HIDs with the oil (might cause them to blow - i'm not sure). This should apply to any oil used.