So far, I have had good luck with the thin liquid super glue. I apply a small amount to the crack and position it so that gravity lets it run down into the crack. I give it a few minutes, then apply another coating. Repeat until the crack is fulled. A finally sanding usually yields an almost invisible fix.
I have also heard of, but never tried, fixing a really bad crack with epoxy mixed with fine saw dust or charcoal (in the case of horn) as a permanent fix with a little camouflage for the ugly spot.
As far as mineral oil goes, I use it a lot. I use it on all of my blades and handles both wood and horn. Not really a soaking, but a good rub down with an oil cloth works well. Mineral oil is nice because it doesn't go rancid, if fairly food safe, and wipes off pretty easily. Another product that folks use on horn handles (especially in dry climates) is Hoof Flex. You can pick this up at most feed stores that cater to horse owners. It helps condition the horn so that cracks don't develop. In my area of the country, I could only see using this product in the winter. In the spring and summer, it's so doggone humid that a horn handle has plenty of moisture to drink right out of the air.