Excellent info guys, thanks.
I recently had an AC breakdown and the humidity shot up from 40% to about 65% in a very short time. All is working now and it's back down to 40%, but it got me thinking.
I'll keep reading, but based on what I'm hearing - and seeing with my own stuff - it's probably a good idea to keep an eye on the leather and wood, horn and ivory. Mineral oil is one of the best tools in my preservation arsenal, I use it on metal, horn and ivory. It's cheap too. I don't think it will harm wood or leather (after all, isn't japanese clove oil mostly mineral oil w/ a hint of clove for scent?), but is there something better to fight the dry air?
BTW, I had an apartment in the UAE that had humidity levels at 80% at one point and I can tell you that NOTHING was comfortable with that. Our clothes got a faint hint of mildew and the leather on my sword scabbards got moldy

. On the other hand, 40% does in fact get pretty dry in the winter and it affects our skin. I'm sure it dries leather and wood as well. My blades like it, though.