It would be a bad idea to put a bunch of WD40 in a hub and then let it sit there, unridden, for many years. The solvent would dry up, leaving the rest drying up as a gluey gooey mass. But it would be okay to put a bit in there and then ride it for a while. How much do you use this hub, and how long have you been using it? If it's an old hub you've only started using recently, then a little WD40 (or other solvent) is a fine idea. If you've already been riding this hub for a good while, and this problem has only just begun, then regular oil is probably better.