I have taken apart my hubs and wondered what type of lubricant I use for the bearings and inside the hub. I have a New Departure Hyatt Model D rear hub.
Wheel bearing grease on the bearings, 10w30 on the brake plates. Some people have other specific brands/weights that they prefer. If you're lucky, you've got one with a port, the brakes need oil more often than the bearings need grease.
While these hubs were designed for use with oil this can be replaced with the use of grade 00 grease (almost liquid). This is not the easiest thing to source but it works quite well. Most generic grease sold for bicycles is a grade 2 which is much to thick.